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.
First thing first
Tec-9 is semi-auto and would have to be modified to become fully auto
also with full-auto you have factors such as barrel climb, recoil, and cycling which make tight streams of bullets impossible
edit:fully automatic weapons are illegal to civilians in most states
and yes full auto weapons are legal to own as long as you pay a special federal tax stamp and register to the ATF
