How are extracurricular activities evaluated for students who have transferred to many different high schools?

That is a tricky situation. I think in your case, showing consistency in your activities school-to-school will definitely help. Colleges will see that as commitment to your interests. Admissions officers should be able to understand that it's hard to be the president of a club at a school you're a new student at. You could always try to run for a leadership position and see what happens though! Who knows, you just might get it.

That being said, I think there are some ways you can get creative! Leadership doesn't need to come from a title. You can try some of these ideas:

Generating impact. With the clubs you do join at your new school, or potentially volunteering outside of school, are there ways you can step up to head an initiative that is very impactful?

Creating your own thing. Is there a club that doesn't exist in your new school? Starting your own club is an AWESOME way to show leadership and get a ton of great experience. Even if it's not a club that you start, maybe you can start a fundraiser or new school event. You could even use your experience as a new student to inspire something - like a buddy system or social event for new students in the school. You've switched schools a few times, so you know what it's like and what helps new students adjust well! There are probably a few other students each year who are new and could benefit from your ideas!