Sounds like she has a couple of big issues going on here. One, she has more than likely begun a "performance career" to cope with peer pressure. She's obviously trying to get acceptance and approval from her peers both female and male at the expense of her academic career. Unfortunately, this click probably has a pecking order and it sounds like the only spot left in the "group" is as "court jester."
She's obviously still to emotionally immature to see how this behavior will negatively impact her in the next 2-3 years. She needs to understand from now until her Junior year in High School is "do or die" academically speaking. I don't know how involved you are in her day to day school work, but I think it is imperative that you make your presence known at school, and schedule a meeting in person if possible, with her English teacher and all of her teachers, to come up with ways to help her improve her grades before the year is out and to distance her from her so-called friends during class.
If this were earlier in the school year, I would even go so far as to have her switched to a different class period for the same class if possible, so that she won't be tempted to be distracted.
I would also encourage you to make your daughter show you her school work for ALL of her subjects every night. Help her if she needs it. If you feel you can not trust her to tell you what her daily assignments should be, get a teacher's syllabus for each class so you know what her daily assignments are, and what she learns each day. Do not let her go to bed until you check off her assignments each night. If you find her struggling academically, and you discover it might be more than an issue of her applying herself, now you will have a starting point for getting the appropriate academic help she needs and before its too late.
As for this "group" or "friend" issue, if all or some of these "friends" are in any of her classes other than English, I'd suspect this is a common theme in all subjects, but for whatever reason, her teachers aren't reporting anything to you. Once again, getting to know her teachers will put them on notice and let them know you're involved in her academci welfare. If you don't have time for one on one meetings, regular phonecalls can be just as effective. Besides, if you make your daughter and her teachers accountable by being involved in her day to day assignments, she'll be less likely to fool around in class, knowing that she'll have you (the provider of her food and creature comforts) to report to, and teachers will be less likely to let her get away with it.
You're right that there is no way to find out if she's keeping her word and staying out trouble while you're at work. I'd guess with an older boyfriend, he's probably been to the house without you're knowing it. Or, she may not even be coming home afterschool like she is supposed to. Plain and simple she's proven she's not mature enough to be on her own without adult guidance. And its clear she has too much freedom and freetime on her hands. To remedy this, I'd first and foremost work on the boyfriend issue. I personally think she's too young for a relationship let alone with an older boy. Her focus should be on academics if she has any aspirations of doing more with her life than being someone's "girlfriend" or "momma".
I'm from the school of thought that strict parental involvement and open communication are the best prevention of teen pregnancy. I think relying on contraceptives to help control a youngster is the "easy way out" of parental responsibility and the quickest way to invite to unwanted trouble.
IMO taking your daughter to the doctor to get contraceptives is the same giving her the green light to get into sexual experimentation. This solution doesn't protect against disease, nor does it protect her from the mental stress that relationships too young can cause, it does not ensure she will perform academically, nor will it ensure that she will stay out of trouble at school or away from home.
You also risk sending her the message that relationships at this stage of the game are equally as important as academics. In reality, her being allowed to be in a relationship this young can only have one certain outcome, no academic future. With time, if you allow her to indulge in a boyfriend, she will put all of her energies into this person and less and less into her own needs. Now is HER time to become the person she's meant to be, and the time for you to reap the satisfaction of helping to develop into that person. It would be a shame to allow some boy to rob this from her, and YOU. In short, forget the shot, and get rid of the boy. Especially one who is too old for her. You're asking for trouble. If necessary, call his parents,or confront him. Tell him you have expectations and that official dating is not a possibility. Only supervised visits if any. If he's the trouble you seem to imply, perhaps more serious action is needed. Maybe threats of a meeting with a police liason from school if he's real trouble? Without more information, I have no better advice except to forbid his presence in her life!!
In this situation, I'd forbid any dating until she's much much older. If you pay the bills, you can forbid dating and effectively enforce this by not making "creature comforts" so easily available. Make her earn her keep and give her chores around the house if she wants a new jacket, shoes, or video game. Until she's old enough to get a job, she should be expected to do chores around the house and make herself useful. She will not respect you or value anything you give her if you don't help to her to understand that in life if you want good things, you have to work for them, and if she wants to coexist happily under your roof, she's going to have to respect your rules and value-system.
Instead get tough on her, talk to her about expectations and consequences. If necessary, maybe its time to talk to her about who she chooses to make friends with and most especially about boys since this goes hand in hand, and set her straight on expectations. Tell her what age you will allow dating. Limit exposure to boys to parent chaperoned group only situations.
If she has male friends, don't allow her to hang out with them until you've had a chance to communicate with the parents of the boy and the boy about expectations. And as I mentioned already, in parent chaperoned situations.
The same expectation should go for new female friends too. It is not a crime to "help" her select friends, and for you to get to know them and their parents. You are her mother out for her best interest. She may not like it, but This way you will have a network of parents who hopefully will help you watch out for your daughter and vice versa. If they won't help, then you don't want your daughter hanging out with their kids. Invite these potential friends to your house on weekends, and make yourself present. Get involved in her activities and with her friends. Do not make the mistake of leaving her to her own devices.
Bottom line I think She still needs supervision and probably until she's much older and has proven that she can be responsible and trusted. I'd suggest finding a way to keep her occupied or "chaperoned" until you get home from work.
It sounds like resources may be a challenge, but I'd strongly encourage you to see if there is an afterschool tutuoring program offered at your daughter's school or within your school district and get her enrolled immediately.
Since it is the end of the school year, availability of district run programs might be slim. Don't forget community ed programs in your area as well, while not free, they're very affordable. There are also similar afterschool programs sponsored by area churches also often free or low cost, run by teacher/tutor volunteers that do the same thing.
If you can't find an afterschool program that is geared academically, get her involved in volunteerism for an hour or two afterschool, she is not too young to put in some constructively spent hours helping out younger kids at a church-run daycare/afterschool program herself.
Last but not least, it is not too late to get her enrolled in intramural/ community sports, arts, dance and music programs. These programs are also very affordable, and run all year round. Perhaps this would also be a great option to keep her out of trouble for a few hours until you get home.
Whether she needs the academic support an afterschool tutoring program would provide or not, or whether she's interested in "volunteering" or not, these are all free/low cost solutions to your soon to get worst problem...all of which will encourage her to be accountable, learn skills that will help her in the future, and possibily keep her out of trouble if she's in an environment with people who care until you are able to be with her.
If this is not an option, I'd consider enlisting a grandparent, or a responsible adult you can trust to help "chaperone" her after school. Maybe a friend from church, a community role-model? This person should be someone who can enforce rules, but not undermine your authority or values. I know this is a big challenge, but I think you're already thinking correctly. Go with your gut, and hopefully some of these ideas can help.