All players are ranked based on ratings, national awards (e.g. All-American team), team success, division, and prestige. Underclassmen then make their decision based on where they project combined with their current class. So a sophomore would require a higher projection than a junior in order for them to leave for the draft.
Also, teams cannot lose any further early entries once they've lost 6 total players (seniors + other early entries). However, players signed as senior transfers no longer count in the number of seniors.