Somehow, someway, Casper High finds themselves in Gotham.
It could be a field trip or a ghost shoved them in a portal, doesn't matter, they're in Gotham.
As they arrive in Gotham, Mr. Lancer decides to turn this into something educational by exploring Gotham and teaching the students its history. To the Gothamites, the Amity Parkers look like children who have never been exposed to crime in their life, never been mugged, never been kidnapped.
But the truth is, compared to the BS Amity is used to, Gotham's issues are like kindergarten. (Poor Gothamites, they're not prepared for the chaos that is Amity natives.)
Or
Amity Parkers go to Gotham and freaks the entire population of Gotham out.
(Notice!! All the Casper High students has a form of super strength, also making it so that they are able to touch ghosts.

They join the ghost brawls every once in a while and has some wins.

Most of you, if not all, are liminal in some way.

Everyone knows that Danny is Phantom but has signed an NDA that says they aren't allowed to tell anyone who isn't a native Amity Parker who he is. Jazz is also an intern psychiatrist at Arkham Asylum.)

Start however you want!