What are the top prestigious schools looking for sophomore year in terms of workload and awards? Is one AP class too little?
Yes, one AP class is decidedly too little. Yeah, sure, don't take AP classes for the sake of it, and don't bite off more than you can chew, but if you're aiming for a T20, I'd at least take 10-12 AP/dual enrolment courses across all four years of high school. Just "challenging yourself" really doesn't cut it anymore in the current college admissions landscape. You need to take on objectively challenging coursework. (Unless you're like an arts person in which case the rules of the game are a bit different for you.)
No worries, though, if you regret not taking on enough courses like I did in my junior year, many universities offer programs through which you can take individual courses online for college credit. If you're into maths/physics and your parents have pretty deep pockets (or you're wiling to apply for financial aid), Stanford ULO may be the right program for you. If you're more interested in other things, I know that Southern New Hampshire University also has a pretty decent program.
Whatever you go with, as long as you receive undergraduate credits for your participation, you can't really go wrong with it. Good luck!
The # of AP classes you take doesn't really matter, just make sure you're challenging yourself and that you can handle the workload. Take AP classes in what you're interested in, if theres another AP class at your school that you want to take out of interest, go for it by all means. Outside of academic, this would be the year to try to start gaining leadership, or at least seting yourself up (I like to use the analogy of setting up dominoes) to become a leader in the club. This will demonstrate to top colleges various skills like communication and teamwork. In addition, it will contribute a lot to your personal development which is valuable on its own. T20 schools don't really look for anything specific, they want students who are passionate about what they do, and these students most importantly are Authentic. Don't do a club or competition just because you think it'll look good to college, do it because you genuinely enjoy it. If you are doing any competition, competing at the state/national level would be really cool, but make sure you're having fun with it.
TL;DR Do stuff you're passionate about, try to compete in state/national level, get some leadership experience, you'll have a great sophomore year!
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Also, beware of sketchy institutions. Don't take courses through an institution that doesn't grant degrees; that's a recipe for disaster. Always double check the accreditation of a university if you don't recognize the name. Regionally accredited is fine, but if it's not accredited your college credits will likely be worthless.