Theories on the Isle of the Lost Schools
“Dragon Hall” is their all-around school, teaching kids the basics of villainy, how to look wicked and alluring, and how to manage your evil army/the tips and tricks to being an effective minion/righthand goon.
According to canon, it’s built inside a crypt with a decrepit church above it, possible Frollo’s old church, or a much larger “All Faiths” institution before the various religious practitioners realized they couldn’t stand each other.
“Serpent Prep” is a specialized school for their aquatic dwellers, or those that simply prefer to be near the sea, like the children of the pirates. It’s got extremely small classes compared to Dragon Hall, and has about 10 to 5 students a class, with a rotating staff of around 7 teachers for the entire school, with a small squad of non-human sidekicks acting as supporting staff.
No clue on if it’s been given a canon location, but my vote is in a crashed ship, half of it sunken into the harbour and completely underwater, with the “Serpent” coming from its figurehead of a sea serpent still standing proud and threatening.
Before you ask: there are probably fences and safeguards against alligators swimming in and snacking on the aquatic students, and Harriet and her crew likely go to Dragon Hall after a worse row than usual that lead to her wanting 6-8 hours of the day away from her brother on weekdays.