The reserves is a part time pursuit. While I understand and respect your eagerness to get in, reserve ambitions only augment the rest of adult life; they supplement it. If you haven't started thinking about post secondary, you need to do so. Without some achoolimg you're pretty much screwed.
As for courses useful as a combat engineer? Math, physics, chemistry, and French. I will emphasize that last one- our military values second language skills, yet most young people blow them off for years and years and regret it later. Don't waste time with Phys Ed courses; keep fit on your own time and use that time for other academic or life skills. Nobody's ever been hurt in their preparation for adult life by a course in money management/accounting, or basic Canadian law. History is also something inherently worthwhile if you can fit it in- know the country you're intending to serve.
