I went to their site to see what their mission statement was:
"Our Mission
Twin Peaks Charter Academy provides students with a rigorous education and the foundations of virtue and character, building cultural literacy through a content-rich curriculum in an encouraging environment."
http://www.twinpeakscharter.org/about/our-values
So the school is wrong in my opinion. Him coming out isn't against their mission statement and the event wouldn't have been any less solemn.
In fact by their bigotry, they not only tried to damage the character of their best and brightest (that goes against their mission statement), they've tarnish their whole school, not just the evening.