J.C.
Why is it that you feel you have to keep him away from her? Is she some sort of evil person? How do you know that she is pressuring him or that her friends are? Why do you automatically assume the worst?
Sit down and have a heart to heart with him. Once you set the guidelines as far as the phone calls (ie he has to use the houseline to make his phone calls to his girlfriend and his cell phone is for emergency use only... or if he goes over in his minutes he has to pay for it either through extra chores or in cash if he has a little part time job). Have the girlfriend over for dinner once a week and see if her parents would be willing to do the same. That way they are getting to spend time together without having to be out of the house without supervision and you get to know her.
Have you had the sex talk with him? IF you have and you have covered all points, then you need to trust him to make the right decision. But don't be foolish...kids that young do have sex so have him go with you to buy condoms. If he isn't mature enough to buy them then he's not ready for sex.
It's not the school's responsibility to keep them apart. They are there for the academics and not the morals part of educating the child. I'm a single mother as well and it's not easy to be both mommy and daddy, especially to a son since we were never boys, but we have to do what we have to do. Just keep the conversation open and refrain from automatically punishing him or assuming that he's doing "bad" things with his girlfriend. If he doesn't feel like he can talk to you... well let's just say that you're more likely to become a granny soon.