Already Tommy had made him self-conscious.

He peered covertly at Tommy, and Tommy caught him at it every time, and then each quickly looked another way, and Cathro vowed never to look again, but did it next minute, and what enraged him most was that he knew Tommy noted his attempts at self-restraint as well as his covert glances.

All the other pupils knew that a change for the worse had come over the dominie's temper.

They saw him punish Tommy frequently without perceptible cause, and that he was still unsatisfied when the punishment was over.

This apparently was because Tommy gave him a look before returning to his seat.