Hi! As said in the title, I’m a high school Sophomore and would really like to get into a high-level school (Brown, Princeton, Cornell, Stanford, etc). I finished my freshman year strong with all A’s in gifted and talented courses, and ended with a weighted GPA of 5.25. Also over the summer I took an online course at the University of Virginia and was selected as the top student of the program. Last year my PSAT 9 score was 1280, which was in the 99th percentile. This year I am taking some harder classes (AP Chemistry, AP World, Gifted & Talented English and Math, Honors Chinese and Creative writing) and have a QPA of 5.57 this quarter. This summer I will be taking part in a foreign exchange trip to China and will hopefully be interning at a Johns Hopkins biochem lab. I think that I will be able to keep up my grades for the rest of the year, but I am somewhat uncertain what to do during the summer of Junior year. I am very passionate about Chemistry, and will be testing for the Chemistry Olympiad, but I don’t see myself getting that far. If anybody has been in a similar place to me I would love some input on how to look better for colleges.
What is your unweighted GPA? Schools will recalculate your weighted GPA and factor in rigor equally to level the playing field. You seem like you’ve done some awesome summer programs! Find something that makes you really unique and different from other students that are applying to those schools. Also take the SAT and ACT (I recommend taking both initially to see which one you do better on) and knock those out of the park, as well as the essays.
I understand that you want to attend a prestigious school since they usually do have a good program for whatever you are planning to pursue. I am going to guess you are interested in chemistry related fields, but you should find some other reason besides the prestige to be interested in a university. You probably understood that, but just wanted to point that out. And junior year summer- you can study for certain competitions and potentially do community service or do other internships or maybe work on research projects. You will find something. You are clearly motivated and interested in your academics.
I agree with jennacwa to focus on SAT or ACT to get them done as soon as possible. If you are taking SAT Subject tests, you should knock out Chemistry since you’re taking AP Chemistry. Start prepping during the year for that occasionally along with your AP prep. Get your standardized tests done by Junior year so you can just start working on applications, junior summer- yeah that is something you should really do then. I’m regretting not starting my college apps in the summer. Haha.
And for Chem Olympiad, it’s possible for you to get to be at most a semi-finalist, but that’s with a lot of preparation. I am not capping your potential, but given you have other academically challenging courses, you will prioritize keeping good grades in classes versus spending more time for Chemistry Olympiad.
Lastly, you should get involved in a club or some extracurricular. It’ll help show that you are more than a book smart person. Colleges in general want a diverse person with wide interests.
I should have mentioned that my unweighted GPA is a 4.0, my bad. During my freshman year I was part of the marching band at my school, ran varsity winter track, and played JV lacrosse. This year I’m doing those again (Hopefully varsity lacrosse in the spring) and am the president of my school’s Chinese club. I checked back here because now I have my course registration for my junior year, and it is as follows: AP Chinese, AP English literature 11, AP U.S History, AP Physics 1, AP Physcs 2, Gifted and Talented Wind ensemble, SAT Prep standard, Gifted and Talented Health 11. I’m hoping to stay in GT Wind ensemble because it will show colleges that I’m more than grades, but it’s tough to keep up with practicing my trumpet sometimes. So far I haven’t let any stress get to me with grades, and all of my extracurricular activities are going well. One added source of strain on me is that this year I will be taking part in a foreign exchange trip to China, so I’ll have to take my finals early this year. Thank you very much for your input so far, it’s very reassuring to have someone who knows more about this than I do! 
Keep earning all A grades in hard courses
Do well on standardized tests.
Have a high level (probably state or national level) achievement in your sports, music, or other activity.
Make sure that your parents are willing to pay what the net price calculator says.