At the first hex, Snape's attention flew to the empty classroom. When he realised the casters were Al and Harry, he relaxed and went back to work.
Or tried to.
Protego felt slithery, like soapsuds, against the stone of the floor. He tried to ignore what was clearly a training session, but then Harry started casting tickling hexes and itching jinxes.
The rotten sod.
The second one that bounced off Al's shield and hit the castle's wall gave Snape a tic under one eye. By the time the fifth hex had been absorbed by Hogwarts' stone, his shoulderblades were twitchy. He sighed pointedly, flung down his quill, and Disapparated to just inside the doorway.
If they were going to distract him from his work, they could bloody well put up with a bit of heckling.