I ended up leaving my old course, and I would be lying if I said that the cliquey atmosphere wasn't one of the things that bothered me. There were a couple of people on my course who I was friendly with, but with the rest, it was like you had to fit a specific model of *insert type of student* and spend all your time with them whenever possible.
I didn't leave solely because of that, obvs. My course wasn't fulfilling and I went through some stuff that I wasn't going to be able to deal with away from home (I'm an international student).
My solution, especially as a mature student, was to spend as much time as I could with other mature students (when possible), as well as other international students. When I had to be around other people on my course, I mostly stuck to the three or four people (out of about 30, like yours) who I was cool with and then was nice but "shy" with the rest.
My hope for you is that they'll grow out of the cliquey attitudes/behaviours, especially if the different years of your course start interacting and mixing a bit more!