Those that won’t behave properly in a mixed setting once they reach marriagable age are clearly not ready to get married.
But they go out, and do get married anyway. And those are the people who either meddle through or end up getting divorced, or the lucky ones grow up and mature with each other (not a bad thing). We don’t want to enable those who should not be dating, and may end up in sin, to go ahead and meet boys/girls s/he would otherwise not be meeting.
In short, who says that people who are clearly not ready don’t get married?
seeing as once a boy and girl have each others name and phone number, they are free to continue their relationship anywhere and anyway they chose.
Same idea with dates from shaddchanim. Although I ahve heard that the boy no longer has the girl’s phone number to call her, so once again, NisKatnu HaDoros.
I think the key factor that makes these events kosher is that all the singles there are seriously interested in getting married
I would imagine that once someone in the class is married, the rest are at least entertaining doing so as well, and would go out for a great shidduch. But you have a good point to exclude friends that are of a younger age, if there are any.