Step 1: Spot the special phrase.
The sentence uses one of the best, which is a fixed pattern to watch.
Step 2: Recall the rule.
After one of the best, the noun must be plural, because we are picking one out of many.
Step 3: Check the noun used.
The sentence says teacher, which is singular. That breaks the rule.
Step 4: Correct it.
It should read one of the best teachers, with an s.
Step 5: Verify the rest.
She is and in the school are both correct.
Step 6: State the answer.
The error is in part C, teacher. \[ \boxed{\text{teacher}} \]