Is your daughter allowed out of school at lunchtime? If not, can you get the school on side and have them spell it out that she's not allowed off site at lunch and will be punished accordingly if she leaves?
I think a trip to your solicitor might me a good idea. i don't know your financial circumstances but all solicitors will give you a free 30 minute consultation. A solicitors letter to the mother of the boy might do the trick.
I remember your previous post about problems with this boy. Both him and his mother sound like total wasters. It makes me wonder if her car is legal, i.e. taxed, tested, insured etc. It's a bit sneaky but how about a phone call to the police to say you are an anonymous concerned citizen?
It must be so hard for you, but keep on being patient and she WILL tire of him.
Good luck.