Check out Wikipedia- (under “Beth Medrash Govoha”)
“An exception to the non-dating rule is that the student may continue to date a girl, if he started seeing her before the semester started.”
It also explains the “or else”-
“If a student is found to have broken the rule, the senior faculty will generally not attend the wedding (which is considered an honor), and there may be other disciplinary consequences, such as the suspension of student from Yeshiva indefinitely.”