Contact the school district main office where she goes to school. Explain your situation and ask what documentation they need to provide you with documentation in writing whether she's still in school. Obviously they won't provide info over the phone to just anyone calling up. They may want a copy of your court papers and a copy of your driver's license.
If they are flat-out uncooperative, you can have a court (same one that issued the child support order) order the mom to have the school district provide proof (directly to the dad from the school district) that the child is still enrolled.
If the child support is being garnished by the state that issued the child support order, he could also ask in writing that his child support case worker verify school enrollment. School districts will confirm this kind of information with child support agencies.