The prohibition is against hearing a woman sing although the word used is "kol" which means voice. It is clearly permissible to hear a woman's speaking voice as is clear from the many times in Tanach and Shas that conversations took place between men and women [Moshe and Bnos Tzlaphchad etc. etc.]. What about a woman lecturing before men?
Interestingly, Rabbi Shlomo Vosner [shevet halevi 3/14] forbids it! There are various reasons. One is that only a normal speaking voice is permitted but not a "public speaking" voice which is a lot more dramatic and hence possibly more stimulating to men.
Rav Moshe Feinstein [או"ח חלק ה סימן יג] allows a woman to give a shiur in front of men provided that it is not in a shul but in a private home and that she is sitting. He also was quite opposed to men and women sitting together during the shiur.
Rav Nosson Geshtetner [להורות נתן ח"א סימן ס] also permitted a women to speak in front of men as the prohibition of kol isha is only if a mans enjoys the SOUND of her voice but not if he is focused on the content.
All of the poskim were unanimous in that they were not crazy about the idea all together. Despite this, in the more modern schools today, female teachers are the norm. Even young attractive ones and even when the students are at the age where this means something. Having to sit and stare at a female for an hour straight is far from being within the bounds of purity. Hard to imagine that this little post will change much but at least it will bring the issue to peoples awareness.
It goes without saying that the practice in at least one high school where the boys and girls study together and the boy is sitting with his chavrusa [or better "chavrutata"] and while she is trying to figure out the Rashi he is trying to figure out how to get her to go out with him [or vice versa - or they are BOTH thinking about the same thing], flies in the face of everything holy and is a disgrace to our holy Torah. May this school use the loads of money it has to spread holiness and not to poison the pure souls of our youth.
A boy shouldn't learn gemara with a girl. Generally it will only lower her level as everyone knows that girls take school more seriously than boys....:-)
In closing it is fascinating to relate that the Rabbi Shlomo Luria [1510-1574] tells in a responsum [תשובות המהרש"ל סימן כט] that he is descended from a certain woman named [Maharat?] Miriam Shapira who was a ROSH YESHIVA and gave shiur to the boys from behind a curtain!
זכות הצדיקים יעזור ויגן ויושיע
[See the sefer Hegyonei Haparsha Vayikra page 576 for more discussion]
