Yes, you are studying in a high school in the US. Yes, you think you’re a competent English user because you’re using it every day. But have you ever been told your English writing reads like gibberish?
Your English might be useful at a Starbucks. In a TOEFL testing room, much to your surprise, it’d be no more functional than your mouse pad.
Be modest. If you can’t, try to be sane. A sound mind is what you’re advised to possess.