First things first, the support network of antenatal classes is fab. Not all branches do post-natal courses, but of course that's another option, as is just going along to coffee mornings/NCT social stuff & making friends there. But antenatal classes are really powerful - you share so much in that 'help, what are we doing' phase.
As lots of people have said, the classes will focus on vaginal birth although they will cover C-section. Are you 100% sure of your decision? Would you like to have more info on other options/are you prepared to hear about the negatives of c-section? How awkward would you feel in a situation where you where challenged/questionned about your reasons. Think these are all worth thinking about. Different teachers use diff approaches ito how they run classes (ime, few will challenge your specific choices -afterall, NCT very actively promotes greater materal choice) but they will all believe that, ideally, CS rates should be lower. That's not to say they will challenge your reasons, but it is the general stance. You need to be prepared for that.
However, as many people have said, you don't HAVE to be honest! One of the girls in my class knew she was going to have a c-section (for medical reasons). We all met up the day before & she didn't say anything - only told us afterwards.
OTOH, we also had one person who was 100% sure she wouldn't b/f (again, not an common choice when attending NCT classes!) She stood up for herself, had definite reasons (emotional, not practical) and I don't think anyone bothered her. She came along to the b/f class (which she could easily have skipped) & asked some controversial (& very very good) questions. If you can go the later route, imho that's better.
Afterwards, no-one will give two hoots- you'll all be too busy worrying about your babies! Before hand, tbh, there may be some judgement - either by the class (most people who attend the classes are leaning towards natural birth in intention at least) or potentially the teacher. BUT I really really do NOT think that any of it would ever be enough to make you feel an outcast - you might just have a few moments of feeling slightly awkward. And tbh, the rest of the class may not even notice as they'll be too busy worrying about their own issues!