Mu daughter is 12 now and in 7th grade and spends most afternoons at home alone doing her homework. She walks to and from school with friends and has a cell phone where she calls me when she gets out of school, and 5 minutes later when she has made it home. It helps that I only work 1 mile from home, but she is very responsible and gets her homework done first thing and then chores. She is a straight A student so I haven't really worried about goofing off yet. I do have some rules, such as no cooking, no friends, no phone calls (except to me and my husband, or a quick call to a friend about homework), no computer and no leaving the house. I am also very strict about the cell phone and it must be put away at 6pm every night and all calls for her must come through the house phone.
So far there haven't been any issues and I feel that she is very safe and responsible. I did start letting her stay home 1 day a week when she was 11 (6th grade) instead of getting her a babysitter. I DO try to have her involved in after-school activities whenever possible so she doesn't spend too much time alone at home (violin lessons once a week, cooking class at school once a week) and she has swim practice 2 or 3 times a week but it is mostly in the evenings and she gets a ride from another parent and I will do the after-practice pick up.
Good luck!