Just think it proves they hated the whole school. The library would typically house the more studious types while the gym would hold the most jocks. Sure it would be wide open but there would be locker rooms, smaller spaces of that nature. That's depending on if they got there fast enough which obviously they didn't. They were focused on the bombs.
I feel that's the biggest reason they chose the library.. directly above the cafeteria.
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"We all gotta die sometime Red." - Sgt Barnes, Platoon.