does it specifically state he has to pick the child up, or does it say his parenting t ime starts after school? if it's the latter then be can have a responsible adult pick the child up for him in his place. even if it's the former, is there something specific that troubled you about his gf ( as in are you concerned for your son's safety). you are going to start a riot if you tell the school not to allow her to pick your son up, so just make sure this is in his best interest
Is there a reason that the girlfriend should not be picking up your son? (like safety concerns)? Consider that this is dad's time and perhaps he can't make his full time - would it bother you less if it was just a babysitter? Also consider that punishing dad doesn't always have the result you want - don't end up punishing your son by limiting dad's time just because you are unhappy with him or hurt. Put your child first and let that be your guide.
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