They should have stopped, and taken custody of anyone who walked out of the school, whether they knew who they were looking for or not. Time to sort that out later. Since there was a delay in the camera they were relying on, it's also possible that Cruz walked out of the school before the cops had even gotten in place outside.
It was reported Cruz ran out with other students. He was wearing a MSD shirt. Not sure how they would have distinguished him from hundreds of other students.