It seems crazy that the school allows that. This is sort of what I mean though, society in general doesn't recognise the danger. No school would just allow strangers into the school to chat with the kids during their free time, or get some flat earth loon in to speak with them, but quite happily expose them to YouTube without supervision.
Lots of schools, though, would allow 12 year olds to read books written by unknown authors or flat earthers. In a world where content is increasingly online, how do you draw the line between protecting preteens from dangerous interactions and preventing them from exploring interesting ideas?
I doubt they'd stock books on Flat Earth in the school library. Maybe it'd get a mention in a book on kookie conspiracy theories alongside Elvis being an alien. There wouldn't be enough material there to get a start then... just keep going indefinitely, like there is on the Web.