By a Shiur Torah or other lecture event:[1]

It is necessary to establish a Mechitza between men and women when giving a Shiur or Drasha. [It is not necessary, however, to have a full blocking Mechitza as is the custom in a synagogue, and rather a mere divider to prevent intermingling suffices.[2]] In a time of great need, it suffices to have the men and women on separate sides without a divider. This especially applies by a Kiruv event.[3]

The Rebbe’s opinion:[4] According to the Rebbe’s directives, a Shiur is to have a Mechitza between the men and women even if the participants are not religious and even if the Shiur is taking place in a nonreligious kibbutz.[5] Furthermore, one is not to participate in a lecture, or a series of lectures, in which men and women learn together.[6] However, in a time of great need [i.e. communist Russia], the Rebbe agreed for exceptions to be made to permit women to be present by a men’s Shiur.[7] Even then, the Rebbe stipulated this on a number of conditions including: 1) The content of the class should be relevant also for women, such as laws that are applicable also to women. 2) That the men and women be by separate sides of the aisle. This will hopefully prevent intermingling, and mixed dancing, and in no way may intermingling be condoned whether explicitly or silently even for this group which is not opposed to intermingling and mixed dancing. 3) This allowance only applies in the absolute mitigating circumstances such as in communist Russia, in which there is no other way to save these Jews from intermarriage. This allowance should not be extended to any other country which do not have these same circumstances, and even in Russia, it may only be done until conditions change.

Children’s event: In the Rebbe’s  presence, it was common to hold children’s events with boys and girls in 770 without any Mechitzos in between [perhaps due to them being below the age of Bar and Bas Mitzvah]. Nonetheless, the boys and girls were on separate sides of the room[8], and a Mechitza was placed during Tefilos.

Yechidus Kelalis: By the Yechidus Kelalis that the Rebbe held for different groups, it was common for the Yechidus to take place in the presence of both the men and women without any Mechitza in between [perhaps due the fact that they remained standing and in a state of fear and trepidation in front of the Rebbe].

Kinnus Zikeinim: By the gathering that the Rebbe held for elderly groups, it was common for there to be a Mechitza between the men and women.

Kaddish: When reciting Kaddish after the Shiur, the men and women are to be on separate sides and not intermingled. However, a Mechitza is not required.

Megillah reading: See Halacha B in end!

______________________________________________________

[1] Admur 315:3 “Such as a Divider that is made at the time of the Drasha to divide between men and women”; Rama 315:1; Taz 315:1; Rashi Kiddushin 81a; Mordechai Shabbos 3 Remez 311; M”B 315:5; Igros Moshe O.C. 5:11 that from the letter of the law a Mechitza is required if it is a Shiur open to the public; 5:12-3; Igros Kodesh 14:445; 26:70; Yagdil Torah Yerushalayim Choveres 11:18; Devar Halacha 15:13;  See Shulchan Menachem 5:80; Pardes Chabad 16:69; Hiskashrus 645; Kefar Chabad Vol. 1050 p. 37; Tiferes Adam 3 E.H. 10; Minchas Yitzchak 2:20 in name of Toras Moshe of Alshich end of Kedoshim

Other opinions: Some Poskim rule that from the letter of the law, it is permitted to have a shiur without a Mechitza between the men and woman side, and the above sources which record such a concept are merely recording an act of piety. [Seridei Eish 2: 8 [1:77 in new edition] that separate seating suffices without a divider; See also Derech Pikudecha Miztvah 35; Shearim Hametzuyanim Behalacha on Kitzur SHU”A 149a; Sheiris Yisrael E.H. 8]

[2] See Rashi Kiddushin ibid; Seridei Eish 1:77

[3] Opinion of Rebbe brought next; Igros Moshe O.C. 5:11; Rav SZ”A in Aleihu Lo Yibol 3:224; See other opinions above that a divider is not required, and in a time of need one may rely on this opinion

[4] See Shulchan Menachem 5:80; Pardes Chabad 16:69; Hiskashrus 645; Kefar Chabad Vol. 1050 p. 37

[5] Igros Kodesh 26:70 in response to Tzach who wanted to give a Torah lesson without a Mechitza in a certain Kibutz due to the demands of the Kibutz, to which the Rebbe replied that the source for requiring a Mechitza is in Halacha as they themselves stated to the Kibbutz leaders, and hence how can they go back on something in Halacha.

[6] Igros Kodesh Vol. 14:445

[7] Rebbe in Yagdil Torah Yerushalayim Choveres 11:18, printed in Shulchan Menachem ibid, “due to the special conditions in communist Russia, I am inclined to be lenient to allow women to join men classes.”; See also Morah Lador Navuch 2:128

[8] See Om Ani Choma 1:24 and Igros Moshe Y.D. 137 that even small children should not be together with the opposite gender