I have several pieces of advice.
First, relax. Your grades are very important for applying to universities. Having good references is also important, and this comes partly from getting along with people, treating people fairly, and keeping up with class work. ECs are less important. ECs are mostly one way that the really top schools (Harvard, Stanford level) decide from among a long list of applicants with essentially perfect academic records. However, you can get into very good universities with very little in the way of ECs.
Secondly, a few ECs that you do very well is just right. You do not need a long list. Think about how to make the Science Olympiad team better for everyone who participates. This might become a very good EC if you can make a success of the activity. The point is to do whatever you do well, and to make the activity better for everyone. Making things better for everyone is really what leadership is about, and listening can be a big part of this. You are not likely to make a meaningful impact in every one of ten ECs – it is just not realistic.
Your ECs do not need to have anything to do with your high school. There are likely to be a variety of outside activities that you could do. A part time job is a very good EC. Volunteering is a good EC. Some students for example get good ECs volunteering through their church or through boy scouts or girl scouts.
You also might want to read the blog on the MIT admissions website called “applying sideways”. This may be written from the perspective of MIT admissions, but applies to any of the top universities in the US. The point as I understand it is to do what you want to do, and do it very well. This is exactly the approach that I took (a long time ago) to get accepted to MIT. Several people that I knew used the same approach to get accepted to very good graduate programs (including Ivy League level graduate programs), mostly after attending a very good but much lower ranked university (maybe top 100, maybe not quite top 100) for their bachelor’s degree.
Of course after graduating from MIT I found myself working with very good coworkers who graduated from a very long list of universities. You do not need to attend MIT or Harvard or Stanford in order to have a good life or to have a positive impact on the world.