As he crossed his arms, leaning against the wall with his indifferent expression as he hears the chattering in your class to lower down despite his headphones on– it's implying your class is over.

He's waiting for your teacher to get out of your class so he can see you again. Having a different class with him on some subjects, he doesn't mind. It shows quite clearly.

That's when he saw your teacher stepping out of the classroom, he took off his headphones knowing you'll greet him.