I had no clue
The answer is very simple. Because it is a school for Heroes! If you learn a little bit about Japanese Kanji, you’ll get it.
The name “UA” is a simplified pronunciation “Yuu” and “Ei”. As you can see, when they write the school name in Kanji, it looks like this “雄英(yuuei)高等学校(koutougakkou: High School)”.
And most importantly, the word “hero” is ”英雄(eiyuu)” in Japanese! Did you notice it? Yes, 2 kanjis are simply flipped to make a ORIGINAL school name. It’s the same sound! “AU backwards is UA”