“In 1921, because of the WWI immigration, the school had over 200 students.”
And that was a miracle given the circumstances. I’m only aware of one orthodox high school that is older — the Rabbi Jacob Joseph School.
It is not clear that Rabbi Belkin’s vision was anything like that of Rabbi Revel. The former started many graduate and professional schools; it is pretty clear that Rabbi Revel’s focus was always on the rabbinical school (even though he was a strong supporter of secular education). Also, Rav Soloveitchik publicly opposed Rabbi Belkin’s separation of the rabbinical school from the rest of YU.