Ethical pondering

A lot of the story I'm writing right now is based on real life, with names changed, and a few details adjusted, plus a few fictional characters. However, the basic relationships between my real characters holds the same in reality, and if any of my friends read this story they'd recognize it for sure. For this reason, none of my friends except for "Betty" even know of my Mibba account, and "Betty" hasn't logged on in ages. The plot of the story will diverge from reality, but it's that that I'm worried about. I'm sure that I'll be showing more than one of my classmates in a really negative light, and I'm just wondering if that's okay. I know they'll never know about it, and that essentially, the characters are not them, but AU versions of them, but still... Is it all right for me to do this? I won't get sued for libel? And legal matters aside, is it all right for me to do this as a friend?