There are many great rabbis that I know who also give classes to women. Every where else I see that guarding your eyes is of crucial importance, but how are rabbis who look directly at women for long periods of time allowed to do so?
Do we say that chinuch outweighs the sin of looking at women?
There are a lot of rabbis that do kiruv and their audience is not always dressed modestly.
Assuming the women are appropriately dressed, the only problem is staring for the purposes of enjoyment. (Here's an mp3 from Rabbi Willig citing several responsa of Rabbi Moshe Feinstein on the subject.) "Lehistakel", not "lir'ot." = "To gaze", not just "to see."
One would hope that a G-d-fearing person applies appropriate judgment as to what's called normal watching of his attendees in the lecture, and what's called prurient intent.
I know of a certain baal teshuva famous rabbi/teacher who was promiscuous before becoming religious. He was instructed by one of the Gedolim to not teach women because of the big test this would be for him. so there's a basis for what you're saying. I think one has to be smart as to what he can handle and decide whether he is able to teach women or not. it's not for everyone. If one finds himself inevitably having "enjoynment" from looking at them, then he should definitely ask advice from a big Rabbi.
Harav Zamir Cohen makes a distinction between looking and examining. Just looking at a woman isn't a problem as long as she is dressed properlly. The problem starts when people start examining. Therefor it's not a sin.
