What should I do if I don't feel comfortable with my classmates?

I am a freshie in college. Which means it's a brand new school to me and I have to start making friends all over again. I made these few friends because at first I thought they were more of the trendy ones compared to the rest of the class so I started hanging out with them, neglecting the rest of the class. However, as we got closer, I realise that they are not the right type of friends for me. I don't feel comfortable at all. For example, all they talk about is sex, girls and even sharing porn websites (ridiculous) . I am the mature and gracious type who prefer classy and down-to-earth people. That's why I don't feel comfortable at all and they don't give a hoot about being real friends with you. I know that walking out of them now will make them say bad things behind my back and ostracizing me. But I really don't want to hang out with them anymore.The thing is, I need a group of reliable friends that I can do project with in class. Being alone will definitely nor benefit me. What do you guys think I should do?