A 29-year-old custodian at a prestigious private school was shot and killed early yesterday while trying to leave a party in Brooklyn, police sources said.

Kevin Roberts, who was engaged to marry the mother of his 3-year-old son, was trying to get his female cousin to leave the party in Bushwick at 2 a.m. when he was shot five times in the back, his fiancée, Sharon Mason, told The Post.

Neither Roberts, a custodian at the Buckley School on the Upper East Side, nor his cousin appear to have been involved in an argument that led to the gunfire, sources said.

He and his cousin were leaving the party when the shots were fired, she said.

“Right now I am just trying to figure out how to explain this to [his son],” Mason said.

The identity of the shooter remains unclear, sources said.