Flex is a great achievement but you’ll need to explain that succinctly on your application.
Ask two current teachers for recommendations (MIT has a good post for teachers about what recommendations should be like). If you can ask American teachers too then pick one of each.
Your Romanian GC would complete all documents (school profile = very important) pertaining to 8-11 and send it to your us GC to upload because your transcript would be divided into two segments in the
Basically due to your flex scholarship it is to be assumed that you got straight As AND are in the top 1% in your country so that should be very clear in the documents your Romanian GC sends.
What state will you be placed in?
What stream or track are you currently in (humanities, stem…)? What classes are you taking this year?
Next year should include 4 AP classes related to what is currently your ‘stream’ and/or core classes. Unlike most exchange students who can fill their schedule with whatever they like you’ll need to take hard classes and will have graduation requirements. (Details may be better PM’'d as they may be too long and personal for a public thread).
What’s your budget for college? Can your parents help in any way? Are there government scholarships at the undergraduate level?