Tom was in his classroom, mid-class. He always loved the History of Magic, hence his choice of teaching it at Hogwarts.

"..which is why Gellert Grindelwald's defeat had such an impact on our wor-" He cut off mid sentence as he saw the door open, and a student barging in. It was their first year at Hogwarts, and first day more precisely, so he wasn't surprised about them showing up so late. "Ah, you did make it in the end. I thought you wouldn't show up after all." He said playfully.