Gossip Girl (2007): Season 2, Episode 17

2 - 17: Carrnal Knowledge

In retaliation for not playing by the unspoken rules of Constance Billard, Blair hatches a plan to bring down her teacher, Rachel. After Chuck wakes up in a hotel and only has a minor recollection of what happened the previous evening, Nate and Vanessa try to help him fill in the missing pieces. Dan and Serena decide to break up. Dan also has sex with Rachel as she is no longer a teacher