We can't use
irregular-at-magic-high-school (30 chars) or
mahouka-koukou-no-rettousei (27 chars). We could go with a shortening of the Japanese title, like:
mahouka-koukou(what I picked)
Or, we could shorten the English title, but I'm not sure that there is any shortening that makes good sense (unless we could just do
magic-high-school, but that omits the important word "irregular").