a) It was easy to obtain legal dynamite at the time, for farming purposes. Restrictions on dynamite were subsequently tightened – largely because of the school attack, IIRC – and that measure seems to have worked.
b) Building bombs is apparently not that easy. Andrew Kehoe was only able to wire the school to explode because he was the school handyman, and even so, half his charges didn't go off. The Columbine shooters attempted to bomb the school cafeteria, which would have killed many more people than the shooting had they been successful, but their bombs failed. (And bombs require a much higher level of premeditation.)
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