Nil Flaps
The minimum fuel you should carry is your:
Taxi Fuel +
Trip fuel, worked out at the planning stage, time A to B x the fuel burn +
Alternate fuel, time to alternate x fuel burn +
Final Reserve Fuel, I would suggest using 45 minutes x fuel burn.This figure is what you MUST land with, if there is any doubt you will have less on arrival its a PAN call, if you KNOW you will land with less its a MAYDAY.
I think this is what your instructor was after.
Your flying order book should clarify what your school expects.