"schools usually don't close" --> That's a good point. If the HM is hell bent as he seems to be about the idea of implementing his system, a one year project would not do... That is a good start as any for "The headmaster, the second generation" something I'd like very much. The problem, as Cartageno says, is that you could spend 300 and the girls would still be attending school. Although, you, rn, should be able to complete everything there is to complete in roughly 80 days.
"Some of the girls could fail and stay on" Maybe, but Cartageno brings another good point, again. Although, the HM, when tricking Mary and Louise, told them Louise should stay and repeat the year, which opens a door for something like that to happen. The incoming problem, storywise, will be that the HM needs to keep his word to Ruth about the girls improving their grades dramatically. Probably the girls graduating and then making cameos could be an option, although a second generation with a couple of girls from this roster would be a nice touch.
Again, is all speculation.