Uniform at school

Hi. Disclaimer: I've never been in the military so if I use some unconventional terms, sorry.

There was a former student that came to my school a couple days ago from Norwich to see some teachers from years past and he was wearing his acu ( think that is what it's called, it's the camo green one that Army gives you). I thought that students had to wear their dress uniforms and could not wear their acu's outside of school? Am I wrong? I was just wondering because this surprised me