This is a pretty good explanation of polite/formal Japanese speech. LingoDeer has a good lil’ chart too. But basically there’s humble and honorific keigo. The former puts you at a ‘lower’ level than who you’re talking to, and the latter puts the person you’re talking to at a ‘higher’ level than you.
Edit: I see you put うち and そと in your headers, which is definitely the actual way keigo is termed, but I’ve always found it easier to think in this way.
In this case, the speaker is using the humble form of です, which is でございます, to put them at a ‘lower’ level than whomever is on the phone. If they were to use the honorific でいらっしゃいます in this case, then it sounds like the speaker is talking respectfully about the existence of the academy which is not themselves (presumably the person on the other end), which is weird and not correct for this situation.
It’s like whenever you enter a store, the staff says some form でいらっしゃいます which is basically saying “You are now at our store you very important respected person” not “we humble staff are here at this store” nope, this isn’t right, see the reply to this post
some edits because for some reason I couldn’t type Japanese for a bit there