ASTRID just finished changing his clothes. He was in the school's infirmary with Snow. After their conversation with Miss Caldwell, the other told Astrid to go to the infirmary to heal his cheek that was starting to swell. Snow volunteered to bring him there.
When they arrived at the infirmary, the doctor was not there. So, he took that chance to change clothes. A few buttons of his shirt came off while he was trying to make himself look like a 'victim'. It's a good thing that he always brought spare clothes with him.
He walked out of the bathroom after he was done.
"Sit, I'll put medicine on you," Snow said when the other saw him.
Astrid had no qualms on that and simply sat down. Snow then sprayed a medicinal spray on his cheek that was probably already swelling by now. He felt the cold touch on his skin and then the stinging slowly subsided until there was no pain left.