Why would he do this?

I had a very random memory pop into my head. Years ago when I was in high school, a man was arrested. He was actually a 30 something year old pretending to be a high schooler. How the school let that happen? Idk. The bizarre thing, while I never really had conversations with him, the kids that did got interviewed and all of them said he was never creepy. He never tried to date any of the girls or anything. I started talking to someone from high school who did have some conversations with him and she said he was really nice. Never creepy. I asked was it nice as in a grooming way and she said no. He just seemed like someone invested in school. I would assume a grown man sneaking into a high school would be doing it to be a predator. But according to everyone he just went to class. Actually turned in assignments(still have no idea how it took him that long to get caught ) and just do high school things. He even joined clubs. If it's true (and I guess we can't know what's going on in his head). But if it's true that he had no harmful intentions and wasn't doing this to groom students then why would someone do that? Did he not like high school and just wanted a do over? I'm just so curious of the thought process