Moving around takes time. If he was looking for maximum victims, he would have started in classrooms and stayed in classrooms, not started in the administration wing, begin a shooting there, which then launched a manhunt (much, much) earlier than would have been done if he started with and focused on only classrooms.
Make any sense?
A single terrorist might not, but a group might want to disrupt a co-ördinated response by creating confusion and removing top responders.