My procedure
I call kids in groups to get their things from their lockers. I send one group at a time. I'll send the next group when only 1 kid is left in the hallway. They all know the rule is to get dressed in the room, then sit in your seat. When they are all sitting and ready to go, then I let them line up.
I do send the slow ones out to their locker first, giving them 10 minutes to pack their things and get dressed. It seems to work for me. Good luck!