Accurate Human Skull Model for Educational Purpose
The best way to ensure you're getting an accurate skull, near as I can tell, is striking the "model" part of your question, and go straight for a real skull.

A cursory search revealed, for instance, Skulls Unlimited International, The Bone Room, and The Evolution Store although other providers probably exist as well.

A regulatory board for model skulls probably doesn't exist, given the apparent ease with which real skulls can be obtained and kept.