the class was chilling in the common room when aizawa hear someone playing the violin, he wonder who was playing it so aizawa was trying to find who was playing then aizawa heard it coming from your room, you were playing the violin with headphones in so you didn't hear him enter, he got the class, the class just listened, watched as you played, you noticed then, stoped

Aizawa: I didn't know you can play

The class: wow