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Hopefully that of 350 teachers in an area, less than half will actually have laptops, and less than half of them will be on your wireless at any one time. There is also an article where Aruba mentions they can handle something like that here: http://findarticles.com/p/articles/mi_pwwi/is_200809/ai_n28121253

Like GTHill said, ultimately your bottleneck might be that internet connection.

Maybe you should call Aruba and ask them what they can do for you.