Well then, same situation here actually, I'm finishing a semester of HS, working two jobs, and doing a co-op at school, so technically 3 jobs (working 12-3 for co-op everyday). So I have two classes, both with a bunch of homework and even though I manage my time rather well (so I think) I still get to bed around 12-2am each night.
Trick is, the more you have on your plate, the more you have to remember. Get a daily planner (agenda?) and write down your daily schedule in that, classes, work times, and leave yourself room for other crap that comes up. Any appointments or due dates? write em in so that during the day you only have one thing on your mind at a time, which is the task you are focusing on, and rely on your daily planner to guide you through the day so that you don't overwehlm your head and get so stressed out. Trust me, I speak from experience on this one.
Btw, I'm 18 aswell.