Good job teaching her right and wrong and enforcing, so you know she's not confused. As you said, she's got lots of reasons why insecurity, loneliness and anger may be festering after a rough few years added to school pressure and a change in age and maturity. Kids this age DO have a conscience. Sometimes they just lack the sophistication to realize it's best not to hurt feelings in the big picture, but they do understand hurt feelings. Right now, she is seeking the negative reaction.
The rules you have set will serve you well once you get this managed, so don't give up because nothing is working right now. Discipline is a fact of life, and is needed by all kids, but rewards are a bit sketchy if directly buying good behavior in the moment. Good behavior should be the norm-it's better that her life is genuinely rewarding all the time in contrast to when she needs discipline. It sounds like you were basically living this way, but with the hurdles you guys have faced, she's gotten knocked off track.
I wouldn't amp up the discipline right now in a crisis (don't get rid of it either) Therapy may help if you are lucky enough to find a great ally for your daughter to talk to who can also give you good advice as a mom for things to do.
What struck me when I read this is that maybe she feels angered or attacked from somewhere, or unhappy about something-maybe people, herself or school, misses her dad, senses something scary to her, something vague- and can't voice it. I think she needs a huge change of routine or scenery and a lot of time with you to re-establish yourself as a best friend as well as the boss. Shake it up so she senses a difference and a new beginning. If there is a way for you to take her on weekend day trips to do something totally new and special and open the avenue for talking by just hanging around together, you may hear some things that are upsetting her. It may take time, but at this age, at least she will want to be with you, unlike a little later.
Over the next few weeks, you should really try to increase time with her and do brand new activities to perk her up. Even if it's stocking up on new coloring books and sitting for as long as she wants and coloring, going through her clothes and editing together, playing dress up, new playdate with new friend and mom you can go to together. whatever. New and special times. Keep up a dialog. Try not to force your agenda. She's almost showing teenage angst. Get a hold of some good books to read together, movies to watch TOGETHER in which girls are heroes and act like ladies- no modern crappy TV shows with their bratty kids. Turn off the tv and have her help with chores while singing or playing games. Just let her "feel you" being there for her in a new and increased way. Increase attention from dad if there's a way to do so if it would make her feel better. Focus on making her life better and broadening her experience and mainly LISTENING for what's bothering her.
Have a meeting with her and let her know you are going to be checking in every week with the bus driver and teacher to make sure she is acting properly. Take her with you to talk to the bus driver and say you're all in this together, and with the teacher too-I know you're already working with them, but make a "symbolic fresh start" meeting with your daughter with the tone, "OK, here's the new and improved young lady, we're going to be checking in, I know she'll act well.." where she is listening to you and seeing them react and getting new reinforcement. Right now she thinks they're already disappointed in her. Wipe the slate, start fresh. Have them encourage her and re-clarify the rules.
When you do have to enforce discipline, like if you personally catch her rolling eyes or being bratty, act swiftly and make sure your tone is not angry. It's not personal. Say it with the tone and motivation, "Its my job as your mom because I love you to make sure you act like the nice young lady you are, so I have to give you a consequence when you do x. You cannot do x, and if you do, x will always have to happen." Then follow through every time. This way, she doesn't sense any anger or personal disdain from you to fuel her anger, nor is she succeeding in upsetting you and getting negative attention so she can "hate you". She is the ONLY one paying for the decision to do wrong actions-her choice. Over time she won't want to continue when the alternative is a nice happy life with enough attention. Don't "get mad and punish her". Be fair and respectful while giving firm consequences if she CHOOSES the wrong action. My kids (4 1/2 and 2 1/2 are at the phase when I sigh like I'm bored and I'm like, "Ah, man, I can't believe you guys are forcing me to do x because you're doing x..." and they're like, "NO WE'RE NOT, MOM!" and they turn it around on a dime like they're outsmarting me, but that's because we've always been very firm and followed through in the past after one or two warnings.
As for rewards-make these your new special activities and her everyday life, don't bribe her to be good. Life doesn't work that way (if ONLY the cops would pay me not to speed), and it can give a false sense of entitlement. Watch out for teenagers who think they should "get something" every time they follow your directions.
Good luck, don't give up, if you keep working hard from your heart, you will be rewarded.