Dude half-laying across me?

My English class read books to preschoolers today, and we were all sitting on a giant rug in their classroom. Out of nowhere, this guy from my class came over and sat near me and put his legs over mine. It was super awkward, but it was a fairly cramped space so I couldn't really scoot away. I ended up pulling my feet back under me when he pretty much sat on them though. Why would he have done that? I thought he was pretty normal until now. And should I have told him off? I really wanted to, but didn't really want to cause a scene or use offensive language in front of the preschoolers.