I'm an international student and my school doesn't offer any AP or Honors classes. I would've self studied for the AP but the exams are already over (I think the last exam was on mid-June).
Are there any alternatives to AP class which will at the very least show admission officers that I am capable of taking rigorous courses?
It depends on what your and your nearby colleges have to offer. If your school has IB courses, then you should definitely sign up for these.
However, if there are definitely not any "advanced" courses in your school, you can try signing up for classes at a local college. At least in the US, most colleges will allow you to sign up for classes at their schools. Dual enrollment (signing up for two institutions at once, usually a high school and college) falls under this category. You could also try contacting professors nearby to ask if you could help with their research.
However, if there are definitely no options nearby, colleges love it when you create opportunities for yourself. You could self-study stuff online alongside your actual academic study. The internet provides a TON of opportunities.
And also, get a good amount of sleep. Colleges want to see that you have balance as well.
Keep in mind that everything you do should be related to your passion. Don't do anything just for the sake of college.
Your best bet is to find some online classes available to you and take some of those. The fact you have a couple certificate courses under your belt along with a 4-month research course with a professor will help you in terms of your ECs.
The other thing to remember is that you are evaluated based on the context of what is available to you. If your high school doesn't offer AP or Honors classes then you aren't going to be negatively impacted by something like that. So if you are able to find something (maybe try Coursera or EdX) to show that you overcame a lack of opportunity that can be beneficial to your application.
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