I as well would advocate staying in school. First off, you mention you'd like to spend a few years in the military, so obviously it isn't viewed by you as a career move. When you do get out, you'll have a tough time doing anything (I'm talking about higher education specifically, but it applies to other areas of life) without a high-school education. And it will be much harder to get that high school education in 3 or 4 years than it will be now. You WILL regret your decision in a few years if you quit school.
Also, I question your train of thought. You say that highschool isn't your thing, and sitting there learning isn't your cup of tea. What do you think University is? University isn't for everyone, and perhaps you're one of those people that it isn't for. I'm not saying you can't, or you shouldn't pursue it, but if you can't even pull off high school then you certainly won't pull off University.
Finally, its important to point out that the Canadian Forces isn't a for-sure employment agency for anybody who wants a job. You've got tests to pass to get in, and it does take time to go through that process. As well, you have to demonstrate during your interview that you're the type of person the Canadian Forces wants (i.e. do they want someone who quits when the going gets tough?). What I'm getting at here is, if you choose to disregard all the good advice you've been given and still quit school, don't make that decision yet. Its a new school year, and its awful easy for you to think back to how nice summer was and how much you don't want to be where you're at. Talk to a recruiter first, maybe even apply, write the tests and jump through all the hoops. Just don't quit school, if you must, until you have an offer letter sitting in your hands.