The NBA's system is so much better in this regard. Underclassmen can declare and go through the pre-draft process but if they don't hire an agent they can back out up until I believe a month or so before the draft and go back to school retaining their eligibility. So many NFL prospects get bad information that inflates their value and until they go through the pre-draft evaluations it's really speculative to put draft grades on them... like ndsubison posted above nearly one third of early entrants didn't even get drafted last year.