A fully automatic gun that is available for civilians, say, the famous tec-9, holds about 20-30 bullets.
With so many students flooding out, you'd have maybe 16 people hit with bullets. Of those 16, about 4 would be hit with fatal shots. Ammunition is expensive so you'd probably hold around 2 clips, excluding the one currently in that gun.
Wasting all your clips you'd kill around 10-12 students by that time. Then, whipping out a sidearm, you'd probably nail two or three more before somebody stops you, everybody has already run, or you're caught and decide to off yourself.
10 People killed is already an achievement.