Okay, I propose this course of action:

1. Arm everyone with guns (except for the Lunatic, probably)
2. Plant an explosive charge on the stair-gate
3. Repeat 2 as we move up the floors
4. Get to the control room.
5. Kick down the door and go in guns blazing
6. Kill the Mastermind
7. Escape the school with more explosives.

Alternatively, just skip 3-6 by planting the explosives at the door. Or a wall.