Someone who does not readily give praise or credit to others

Solution 1:

Not a single word, but your question reminds me of a teacher I had during my fellowship who was described by everyone as "hard to please".

• hard to please, demanding, requiring more than usually expected or thought due; especially great patience and effort and skill. TFD

From the web:

• "...trainees knew him as a hard-to-please teacher who expected them to work as hard as he did."
• "his father was a hard-to-please taskmaster."
• "They all think a great deal of him because he is so hard to please, and almost any of them would feel immensely flattered if he liked them."
• "How spending second grade with a hard-to-please teacher may end up teaching my daughter one of life's most important lessons."

Solution 2:

I don't think there is a single word for someone who is reluctant to give praise or positive strokes, but an apt expression for the type of teacher you describe might be:

tight-ass.

This word has too senses: "mean, miserly, ungenerous"; and, "rigid, repressed, or conventional". A teacher who only gives praise for a perfect performance is a tight-ass in both senses.

As a bonus, it is similar to hard-ass, which describes many martial arts teachers.

Also, although you'd have to give some context (she's a tight-ass, when it comes to giving praise), that's true of the word in general, because of its inherent ambiguity.