RANDOM FROM 0.1+0.(Brains/4) + 0.(Luck/4) + (Activity ranging from .1-.5) 

TO 

0.5+0.(Brains/4) + 0.(Luck/4) + (Activity ranging from 0.1-0.5)

Basically, the more active you are in the class the higher skill you will gain. The persons brains depends how quickly they can pick up the skill, and luck plays a part in everything someone does.