"Ona byla špatnou učitelkou."

Translation: She was a bad teacher.

[L0k4ss0]

Is this sentence using instrumental case? Why? Would it be wrong if I said "ona byla špatná studentka"?

[Nemesis_NaR]

I suppose you meant "Ona byla špatná učitelka." That is also correct and a little bit more common. However Czech really can use instrumental case for humans when talking about jobs and functions as well:

"Jsem starosta Brna/Jsem starostou Brna"

"František byl vášnivý fotbalista/František byl vášnivým fotbalistou."