It's not high school, it's just two or three months later towards the end of the first year of middle school. Kao is still tiny, she'd be on the cusp of entering middle school if this was a three-year timeskip. Komichi is still wearing her middle school uniform too, so is everyone around her and she's clearly not older than them. Erika most likely reached some agreement with her dad at the festival such as being allowed to stay at Roubai for high school but having to be as serious as possible about playing piano and thus having to sometimes travel around. It would be very odd if she just failed to convince him and hadn't been around the whole first year of high school when the play apparently went so well.