That's how Caltech got from about 15% female undergraduates when I was there (early '80s) to 44% female nowadays.
They treated qualified high school girls the way schools with big highly rated athletics programs treat star high school athletes. They would identify prospects, and then put a lot of effort into convincing them they wanted to come to Caltech.
For boys you only got more than lukewarm recruitment attention if you looked like you might be the next Arthur Rubin or Peter Shor.
The girls that did apply were judged using the same standards that were used for the boys, so they didn't have any advantage or disadvantage in actually getting admitted (except possibly in the case of ties...not sure how they handled those).