Yeah, because a teacher is a sentient being with volition and not a tool under your control following your commands. It’s going to be hard to rule the tool deliberately helped him in planning it, especially after he spent a lot of time trying to break the tool to work in his favor (at least, it’s what is suggested in the article, and that source doesn’t have the full content of the chat, just the part that could be used for their case).
I guess more mandatory age verification are coming because parents can’t be responsible for what their kids do with the devices they give them.