If you're 12 years old, then you were probably vaccinated for varicella (chicken pox). It would be unlikely that you get chicken pox from going over to their house and sleeping over, but you might want to have your mom check your vaccination record first. If you've been vaccinated, then probably okay, if not....then stay away for a while.
Chicken pox becomes non-infectious when ALL vesicles have scabbed over.