Training for the NFL is a full time job with so much of the evaluation coming down to specialized events such as the three cone drill and 40 yard dash. When James Williams and Hercules Mata'afa went pro early, both felt they were getting drafted by at least the 4th round and probably would have dropped school all together.
It would be helpful to have the NBA's declare but don't sign rule to allow teams to give honest feedback on where they see a kid on the draft board with some tips on how to improve their draft stock. NFL careers are short and players want as many years of getting paid as possible, it's always going to be a hard sell to convince kids who are ready to get paid to stay in school.