I've joined a couple of U3A classes in the past. I've found them cliquey too, but not everyone there was the same of course.
I've been shouted at for sitting in someone's chair [not knowingly], and was the 'baddie' from then on, dirty looks from then on! In another group, as I stood there, I was blatantly left out of a discussion for a Christmas lunch with people I'd mixed with for months with no problems...I obviously wasn't included!
I then witnessed a new married couple be ignored and side lined when they spoke. Ok they were a little different in their dress sense, but were both very intelligent, decent people and interesting. It was so rude. They eventually left, and I did too, disgusted.
Other things happened, I won't go on, but it's not for me. Too much snobbery in this area.