Gre is the postgraduate entrance examination in North America, and the doctoral degree in the United States is more than five years (science and engineering, sometimes delayed graduation). Generally, scholarships are given according to your research in a certain field, especially doctors from Harvard, ucb and Stanford. You need a deep theoretical and practical foundation in this field before you can give you an offer~~
I haven't finished high school yet, so it's obviously useless to take gre. Undergraduate students focus on English and comprehensive quality, and graduate students need to pay attention to professional level. So if you want to go out and extend your scholarship when you are an undergraduate, you can only get higher sat and ibt, which is unrealistic for you, and I believe you don't want to live a boring life like phd as soon as you finish high school ~ ~
Finally, I'll give you a suggestion. Learning English well is the most important thing.