It seems, unbeknownst to Shiina her powers are increasing (doing some healing magic on Mimi's bruised head, giving life to the plant). For a moment I wondered if the roommate had secretly performed the spell on Shiina -- but then I recalled Shiina having had a 3-days-old injury the day after her roommate died and also losing an arm...
i don't think they are increasing, just that they are in a school to teach them magic for war, and that is not where her powers lie. she has been getting measured by the wrong standards. it's like an autistic kid who's super math genius, but can barely string two words together if talked to. so you put them in a class for public speaking, stick em in front of everyone and tell them to recite poetry, of course they'd fail.