Question: A few years ago, my son came home from his elementary yeshiva highly annoyed that his rebbe referred to another neighborhood yeshiva as "not a **real** yeshiva" because the other yeshiva has mixed gender classes.

When I went to elementary yeshiva several decades ago, it was a mixed-gender yeshiva and boys and girls were in the same class. Since about 30 years ago, it seems that most yeshivot are single gender. As far as I know, a yeshiva is just a place of Jewish learning and I'm unaware that there is any requirement that it be single gender.

Is there some halacha or minhag that actually does require a yeshiva to be single gender "completely" - meaning all students must be of the same gender? Or, can a yeshiva have both genders but they need to have separate classes for each gender? Or, as my yeshiva had, can both genders be in the same class? Does a student's age matter in these decisions?

Please focus the answer specifically to halachic or minhag requirements. I am not discussing the advantages or disadvantages of having a specific system.

Answer: 
In [Igros Moshe YD 1:137](http://www.hebrewbooks.org/pdfpager.aspx?req=917&st=&pgnum=273&hilite=), R' Moshe Feinstein writes that although at a very young age, the issues of *hirhur* and *yetzer hara* that generally rise with co-ed schools facilitating pubescent teenagers mixing with each other do not apply, nevertheless 'אין רוח חכמים נוחה מזה'.

He deliberates whether this (for *young* children) is *halachically* forbidden due to Chinuch concerns, or if it is just unacceptable practice although halachically permitted, eventually pinning this on a non-related *machlokes* between the *Mishne Bruro* and *Magen Avraham* regarding the permissibility of allowing a child to take a set of non-Kosher ד׳ מינים, which he explains to be a fundemental difference of opinion in how to define the obligation of Chinuch.

He seems clear that by older children, where there already is an issue of *hirhur* and *yetzer hara*, co-ed would be unequivocally forbidden.

(It seems obvious that the prohibition is not having two genders registered in the same institution, but in habituating the mixing of the genders together. If measures were taken to make sure that there was no socializing or integrating between the genders, then it would be permitted for them to use the same campus and facilities, as long as they were kep seperate.)
Rabbi Moshe Feinstein speaks about this topic numerous times in Igros Moshe 
(like inYore De'ah 1 siman 137, Yore De'ah 3 siman 78) and he says that even from a very young age boys and girls should study separately but if this isn't possible then can be done. However from an older age it is "Assur min hadin" according to everyone- שנת השביעי ושנת השמיני... הוא אסור מדינא לכולי עלמא וכו' .
ומרוב הפשיטות אין להאריך.