I did UMEP Japanese last year, which I think would be somewhat similar to UMEP French. First of all, there are only THREE students did UMEP Japanese last year from the whole state, I was a little bit surprised to be honest. This course is apparently for those who has complete VCE Japanese 3&4, but the contents taught is way too easy, everything taught in Semester 1 is also taught in VCE, and Semester 2 is also very very easy. I know I shouldn't be complaining it being easy, but I have learned absolutely nothing in doing UMEP Japanese, but it's probably because I also had self taught Japanese for like 5,6 years before I took this course. The two other girls who did UMEP Japaense last year, one is born in Japan or something like that, one went on exchange to Japan for like a year or something, so they all found it ridiculously easy. Back to topic, so, yeah, in each tutorial, there were like around 15 people, and only 1 or 2 of us were doing VCE. But also, surprisingly, also I was in school uniform most of the time, no one really seemed to notice or care. That's one thing about uni, no one really cares about how other people dress, so it's not awkward at all. As in making friends, yes you absolutely can. This should be the same for every languages, you have to work on some sort of group assignment, and you do have to meet regularly and you certainly can and will make some friends.
So don't be nervous! If it is the same level as Japanese (Japanese 3 & Japanese 4 at UoM), the stuff would be easy for one who has completed VCE French. There won't be that many VCE kids, but dressing in school uniform is definitely not weird. And it's very likely you'll make some good friends. The downside is, don't expect you'll learn too much, treat it as something that relaxes you from actual VCE subjects.
Good luck!