Premise is absurd

Between the first and second movies Deloris has gone from nobody with nothing to Las Vegas starring act. That means enough time has passed for something like that to happen. Months, if not years. During that time, I'm guessing that the highly cloistered nuns of St. Catherine's decided to get out and do more for their community and interact more with people. They were taught (by Deloris of all people) to do more good in the world and be more active and engaged.

That desire would get them looking into what they could do, they likely settled on teaching. Many of them likely had skills and knowledge sets that they could impart (math, history - I don't remember what they're supposed to have been teaching, but whatever). Do they have qualifications? Maybe. Maybe some do. Does it matter with a Catholic school? Are they just allowed to teach automatically if they're nuns and/or priests? I don't know.

They'd likely be given the opportunity to help because the Pope himself visited St. Catherine's, so if they asked to be given a chance to help out with a school, they'd get it.

I don't remember if any of this is contradicted in the film, it's been ages since I've seen it.

As for Deloris, if they said she was a nun, and if nuns are allowed to teach at a Catholic school, who'd question it?

Also worth considering: at such a run-down school, I doubt if any administrators care enough to check, so they could be up to anything. Clearly nobody cared about that place except Deloris and the nuns.

Finally: it's a goofy movie and the premise can be a little B.S. as long as the movie's fun. And it is. It's corny, schlocky fun. It's nowhere near as good as the first one, but it's kinda charming.

Technically, of course, nuns are cloistered, religious sisters are more involved with the secular world. However, many members of the public tend to identify all female religious as nuns.

Both nuns and sisters, generally, live in convents. That's a fairly catch all term.

I certainly do not have any issues with the them becoming school teachers. Religious sisters are often school teachers. Nuns are also transferred from one assignment to another. It is a stretch that all of the sisters would go from one assignment to another, but that is a common trope for these kind of films.

This has to be a troll.