I studied abroad in Indonesia in high school, and I highly recommend it. Unfortunately I can't say much for Thailand since I've never been and don't know too much.

I lived in Jakarta with a host family. It was quite a big change from my college town life upbringing in USA. The city was fine but I must say I personally enjoyed being in the smaller cities like Yogya, Bandung, Solo, and Bogor. Jakarta is incredibly congested and polluted, it can take hours to drive a single mile in town and I can count on one hand the number of days I saw the sun thru the smog. Since you are rural it can be a pretty big culture shock so do take that seriously. It's still a great choice though!

It's true that Indonesia is quite cheap. I'm assuming Thailand is too if you do your shopping out of touristy areas. Both are going to be hot and humid but Indonesia dresses fairly modestly (dunno about Thailand) so you might need to buy new clothes either way.

Since you are an American high school student I'm also going to direct you to the YES Abroad website, it's a full scholarship paid by the US State Department to study abroad for a year in countries with Muslim populations. Indonesia and Thailand are both options :) I hope you will consider it as it's a great opportunity. I didn't personally do it but I know a lot of people who did and truly enjoyed it.

You're free to message me if you have any questions about studying in Indonesia!