I don't have time or energy to go into the philosophical and empirical reasons monogendered classes aren't good educational policy. Rather, I'll regale you with a secondhand anecdote.
Back in the day, I attended LeTourneau University, one of a few English majors in a largely engineering school. One of my history profs, who had been there since the school had a history department (when it was called "LeTourneau College"), described what life was like before the school went co-ed.
"Guys would slouch, sleep, act pretty much like slobs, even belching and farting in class," he told us. "When the first girl came, it was magic. All of a sudden, the boys are men, not just men, but perfect gentlemen. They're holding doors, sitting up straight, quiet and attentive. Everyone's trying to impress that one girl."
(I once took a calculus class with seven other guys. We ended the school year with a footrace in the parking lot.)
No, I'm not trying to perpetuate gender stereotypes. In these days of post-feminism, girls are just as aggressive and squirrelly and likely to belch in class. But there's something about a classroom full of testosterone and sweat. Something scary.