Not a classroom setting, but I recently needed to investigate a software engineer in my team that has allegedly been using ChatGPT to do their work. My company works with critical customer data, so we're banned from using any generative AI tools.

It's really easy to tell. The accused engineer cannot explain their own code, they've been seen using ChatGPT at work, and they're stupid enough to submit code with wildly different styling when we dictate the use of a formatter to ensure our code style is consistent. It's pretty cut and dry, IMO.

I imagine that teachers will also do the same thing. My wife is a teacher, and has asked me about AI tools in the past. Her school hasn't had any issues, because it's really obvious when ChatGPT has been used - similarly to how it's obvious when someone ripped some shit off the internet and paraphrased some parts to get around web searches.