Maybe he did intend it to be totally educational, but what about how it may have been perceived by other students? He can control his intent, he cannot control others' perception. Los Angeles is a pretty diverse place, and its very likely that other students of different backgrounds may have been frightened or offended by such a display, especially in the current national environment.
one 14 year old in an obviously fake Klan outfit terrifies someone?...even more so in a place that is very diverse?