a 4* good academy player can gain a +170% in 3 bars/practice while a 5* fast trainer can gain a 110%.
This extra gaining of the academy player if going to quality skill points, it's useless because he lack of 70-90 sp from the 5* player.
But if this extra gaining is going to increase a super duper special ability (that works too), it could be interesting to keep him even as a 4* player.
That's my thought .