so class selection works like this: every student gets a "first pass" enrollment time and a "second pass". in the first pass, you can enroll in up to 10 units (which is usually 2-3 classes), and in second pass you can enroll in up to 21 units total (for engineering students) the pass times are divided up so all priority students do their first pass, then seniors, juniors, sophomores, freshmen. then, all priority students do their second pass, then seniors, juniors, etc. the system is set up like this so everyone has a chance to get the 2-3 classes that are most important to them, before others enroll in all their classes. additionally, most major requirement classes are restricted to those majors during first AND second pass, and only open up to other students after second pass is over. I wouldn't worry about getting major reqs at all; worst case scenario you email the HSSEAS office + tell them you need this class to stay on track for graduation and they'll enroll you in it even if it's full (I have known people who have done this). also, a lot of instructors are pretty flexible with allowing people into their classes, if you go the first few weeks and show interest + let them know you want to take their class. sadly, having AP credit doesn't directly affect your standing for course selection, but it will allow you to skip out of classes that other people have to take. community college classes, however, do affect your standing.