How can I get over the guilt of what I've done?

[?!]

Ok, I'm pretty much just a typical 17 year old girl. I'm pretty popular, I'm a cheerleader, and a senior.
Honestly, I was really just trying to have fun. There's this super quiet girl (let's call her Beth) in some of my classes. She hardly says a word, only speaks when spoken to. Even though she keeps to herself, everyone knows who she is.
Well, my friend told me she over heard Beth talking on the phone saying something about "her son" during lunch. I asked Beth later if she did have a son. She looked kind of surprised that I knew this, but didn't say anything. My friends and I gathered around her in class and demanded she say yes or no. She just nodded and started pretending to read.
My friends and I sat close by her, saying who on earth would want to "do her" she's so quiet and probably no fun, and wow she's probably slut.
One day she walked in class late, and I called out "Here comes the slut!" Everyone laughed and the teacher didn't say anything. Beth looked like she was about to cry, so I didn't say anything else.
My friends and I, and some other people who wanted to join in on the fun, kept calling Beth a slut and a whore every day, and even passed her notes calling her this in class, and trying to stick notes on her, but the nice kids kept taking them off for her.
I would tell her things like, "don't even try to steal my bf, he doesn't like sluts!"
She always got teary eyes and would pretend to read, but it was obvious she wasn't reading because she turned the pages too fast and her eyes weren't moving.
Finally on Tuesday, I asked "How old is your son?"
She said, "Three."
We all gasped and I was like "So you were 14 when you had him?"
She nodded, and I said, "Then you were probably like only 13 when you got pregnant! You're such a slut, even more than we thought!"
Then she said quietly, "I got raped."

None of us said anything after that. I felt so horrible! Then she didn't come to school Wednesday or today! I told my friends how I felt, and they said it wasn't our fault because we didn't know she was raped. Is this good enough reason to not feel guilt? How do I get over this?