You are in a really tough situation. She is young to be in 7th grade and all the older guys are going after her - that's scary. At that age only group dating is appropriate- if any is allowed. The teachers can't keep the boys away from her. And she is probably enjoying the attention, which makes it even harder for her to listen to you
the good news is that at this level "dating" usually lasts about two weeks and then it moves on. Keep her really busy in sports. The self confidence that comes from sports and academic work will help out in many ways.
You also might want to read, Boys Lie – How Not to Get Played by Belisa Vranich, Psy.D. and Holly Eagleson. and then decide if you want to read some of it. If not keep it around for high school.
To some extent you have to sit back and hope that all of the good work that you have been doing for the last 12 years will pay off. Its hard. You can't control what goes on at school, but you can lovingly control afterwards. Do be fair, but don't be afraid to draw boundaries. Best wishes
I would let her teachers know, so they can keep these boys away from her. Tell her to "break up" with the boy she's going out with, and to ignore them. Tell her to stick with her other friends so that they can keep those boys away from her.
Hope I helped!