Saying "an AI detector" is a bit like saying "an AI" - there's a pretty big difference between Llama 3B and Opus 4.8.
Anything beyond that - even telling your AI to go use a "humanizer" skill in your prompt - creates a text distribution that is functionally indistinguishable from human generated writing. Throw in the fact that people who use AI will be influenced by the writing, often in positive, beneficial ways, and the software becomes a vicious, punitive tool. It's worse than waving a dowsing rod or pendulum, because the software comes with the implication of legitimacy and fairness.
I've tried it on large docs I've written well before the AI times, and that are nowhere available on the Internet (so it can't be a corpus issue) - and it is happily classifying me as 60%-80% AI.
It's too bad the teachers can't teach.