I am taking an elite summer program called Summer STEM Institute where I will be working on research projects with college professors and technology company entrepreneurs. Is it better to ask for a recommendation letter from my research mentor instead of my main subject high school teachers?
BOTH! Depending on the school you could be allowed to submit even more than three letters, though you seem like you might be aiming for a competitive school and they usually limit you to three. However, it is always good to have different letters to chose from. However, don't get the recommendation from your mentor because they come from a prestigious institute; get the recommendation if you think they know you well and could appreciatingly describe what you have to offer.
If the recommendation letter cap is of 3 letters then the right thing to do would be to ask for a recommendation letter from the person who knows you well and can vouch for you without being vague. Does your research mentor know you well enough to write a recommendation letter(Mentioning specific qualities about you with explaination)? If yes, then go ahead and ask them. In the end, qualification is secondary to the quality of the recommendation letter. Much better to have a recommendation letter from a High School teacher who knows you well than a noble prize winner who has no idea who you are.
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