Thriller category, a group of friends was hiding a girl who had seen them commit a crime

So I remember this movie (this is not the beginning) where some friends were at a party (in the house of the boy that I will talk abt I believe) and then I think someone got killed, but not on purpose, and one girl had seen what happened so the group locked her in a room in the house of one of the friend group members. This friend was a boy he had a big house and his dad was like a lawyer or something(he had an important job). What I can remember is that the girl who got locked up had a brother and a mother who were looking for her , they tried to unlock her phone but they didn’t know the passcode. They called the police and an officer, not convinced that was the right house, went to the house where they were staying and when he asked to enter the house the boy said no by reminding him of his father’s job and so the officer went away. I think that after they threw the girl into a lake but her mom and brother got there in time to save her(?) The police caught the group (I remember there was a blonde girl) and in the end they were celebrating the girls bday. It’s all I can’t think of.