hard taskmaster

He had been a hard taskmaster, but a concerned one.

Already he was acquiring for himself the reputation of a hard and even brutal taskmaster.

Dan was a quite hard taskmaster in the studio wasn't he?

But many an idealistic young man has turned out to be a hard taskmaster.

He was a hard taskmaster and set high but achievable standards.

In some ways she was a harder taskmaster than you are.

I imagine she was a hard taskmaster, or should that be taskmistress?

True to his word, he schooled her in horsemanship and was a hard taskmaster.

In addition to his photojournalism assignments, he was a patient teacher but hard taskmaster.