When I was last in school (about 10 years ago), it was whatever the difference is between the student price and full face value. For instance, if a student ticket was $4 and general admission tickets were $30, you had to pay $26. Those prices are outdated, but you get the point.
Also, this conversion had to take place Mon.-Fri. at the ticket office in Coleman during the week of the game. If I remember correctly, they actually give you a different ticket that has full general admission value. The conversion money cannot be paid at the gate the day of the game.