I was wondering if I could say "a giving teacher", but I couldn't find that word having been used in such a context. Also, I have the word "generous" in mind, but I think that word has a positive connotation, which I'm not against, but are there any alternatives?
I can't think of a single word that has both the meaning of "generous" with the connotation of "inappropriateness in being generous".
I'd say "an overly/excessively lenient/indulgent teacher" so as to communicate the idea of excess irrespectively of any notion of evaluation system.
To make clear that from the standpoint of the evaluation of the student's worth that one expects from a teacher, the overly lenient teacher is not fulfilling his/her duty in being lenient beyond measure I'd say "an inordinately lenient/indulgent teacher".