Is this sexual assault?

I was in my Biology lesson, and a guy was sitting next to me. I was minding my own business, but all of a sudden, the guy next to me grabbed my boob. I was in shock. I was speechless, and I didn't know what to say. I'm generally a very shy person. I looked over at him, and he was just sitting there, working. He didn't say anything to me, he didn't even look at me. He just didn't react.

Maybe it was an accident, it's happened to me as well. Where I walked directly into a female and felt a bit of boob rubbing against my arm.

I was thinking that... but he grabbed it.