I am a freshman attending one of the top private high schools in the nation, so my grades are low compared to others in my class. I currently have a 4.2 GPA, what are my chances to attend a school such as UCLA based on pure academics? Also, I just completely failed a Chemistry test, am I stressing too much over this?
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Worrying about will happen in 3 years is indeed a distraction from worrying about something you can do things about right now (such as chemistry).
Off topic, but there is no causal relationship between being in one of the top private high schools and having “low” grades compared to others in your class: the people who have “high” grades in your class go to the same school.
There is no way to ‘chance’ a student for any college based on 1 semester of 9th grade, but fwiw, the UCs only use grades from grade 10 & 11.
Read this from MIT admissions- it applies to pretty much any top school:
http://mitadmissions.org/blogs/entry/applying_sideways
Then go talk to your chemistry teacher 
It is way too early to think about specific colleges (especially the hyper-competitive ones). You don’t even have one full year’s GPA and you have no standardized testing. You also need to recognize that HS should be an experience in and of itself – a time of learning and growth and not just a 4 year college application prep experience.
It is good to take school seriously and know that college will be on your horizon, but it is too early to start planning for specific colleges. I would highly recommend that you get off of CC until your junior year.
For now you should focus on:
–Working hard, learning, and doing as well as you can in the most challenging curriculum you can manage.
–When the time comes study for standardized tests.
–Continue your involvement in activities you care about and work towards making meaningful contributions to those activities.
–Enjoying spending time with your family and friends.