So I’ve been getting low As and Bs my entire high school career. My weighted GPA is a 3.9. My SAT is a 1990, and my ACT a 28. I have over 300 hrs of volunteering. And I work. I have excellent teachers recs too. However, I took honors chemistry my junior year and I ended with a C. How does this look on college applications and how will is affect me? Can I still get into umass Amherst, BU, Purdue? Thanks!
Also I want to major in bioengineering or computer engineering!
Though the C is definitely going to detract from your application, it’s not the end of the world, as it is only one grade. Remember, the app process is holistic for those colleges that you listed. Keep your grades up (no more C’s please), continue to pursue your EC’s, write some killer essays, and hope the odds swing in your favor, that’s all. Don’t sweat it. 
Just don’t kill yourself over one lower grade- its not worth it. and keep in mind that the top 20 schools are SO difficult to get into, and there are so many other colleges that are just as amazing but not as selective. I don’t think one lower grade will hurt you very much, except at the most selective colleges.
Did you take the SAT subject test for Chemistry?
no should i