Hmmm, well I know a few people from private schools at Mildert, though admittedly not many compared to comprehensives, but I fail to see why more people being from comprehensive schools would make Mildert any worse? People should never be judged by what school they went to. And I don't recall any vast amount of "Mildert only" events last year, in fact I don't really recall any besides the usual things, so that can't be an issue any more.
Anyway, to the person considering leaving Mildert before they've even arrived: Mildert is amazing, give it a chance and you'll love it. I don't know anyone who didn't love their first year. If you go in with a closed mind, you can't expect to enjoy it. Just meet people, and try to have fun and keep busy, and if after a couple of weeks you don't like it, then move. You're probably just nervously hoping it's going to be okay, and because you were sent to Mildert you're having doubts, but hardly anyone doesn't end up loving the college they were put in if they didn't get in to their original choice.