I think validating her is important, yeah, but there's something we sometimes forget: you also need to make it clear that what her classmates did isn't acceptable, you know? It's not just about her feelings being valid (because they are), it's also about her knowing that comments about other people's bodies are wrong. The difference is subtle but it matters. A friend of mine went through something similar with her daughter, and what worked was her saying something like "you didn't deserve to hear that, and whoever said it was being disrespectful" - not like she was demanding action, but like she was standing by her side.
After that, then it's worth exploring what she wants to do. Ask if she wants to tell someone at school, if she wants strategies for responding if it happens again, or if for now she just needs you to know. And about the pool - that step back is important because it shows she's avoiding situations. Maybe it'd be good to go back, but not under pressure. Like, "I get that it's uncomfortable, but you deserve to enjoy the class just like anyone else". You two could even plan how she'd feel more comfortable - whether it's about what she wears, when she gets in, whatever.
The thing is, at 14 the pressure about bodies is absolutely intense, and we tend to think it's no big deal because "everyone goes through it". They do, but that doesn't mean it's okay to normalize it. She needs to know that you believe her body is normal and that whoever made the comment is the one in the wrong, not her.