2scents — It seems you are more familiar with the boys yeshivos than the girls sems. There is a LOT less hefkeirus among the girls seminaries. Most have dorms with strictly enforced curfews and madrichot living in the dorm and an aim bayit either in or right near the dorm. Attendance is taken at classes, and students who are AWOL one to often are accountable. Meals are generally provided all week, and some Shabbosim, with assistance for girls who need to find places on weeks that the dorm doesn’t provide Shabbos meals.
Seminaries tend to do VERY thorough checking on their students, so it is unlikely that there will be “very bad students” in a seminary that is catering to “good” students. There are also seminaries who cater to girls who want more freedom, or who are looking for more “fun” then than learning, but if you are going to one of those seminaries you are most likely aware of what you are going for.
I am not denying that a certain number of young people get involved in inappropriate behavior when living away from home for the first time. But your statements that “MOST” young people learning here change for the worse is just not supported by any facts whatsoever. There are literally thousands of seminary girls and yeshiva guys here from Chutz L’aretz, and a huge percentage of them end up growing tremendously in their avodas Hashem. (I was one of them, and I had many close friends who had similar experiences. I also had a few sisters who came to Eretz Yisroel as well, and my mother is involved in Chinuch Habanos in Chutz L’aretz and has had literally hundreds of students who attended seminaries in E”Y. So although I don’t claim to be an expert, I have enough first hand experience to find your generalizations to be wrong.)