Looking from the outside I can understand their position. If they are collecting a bunch of kids at the same time, one child who causes a problem and has a tantrum takes their attention away from others and has the potential to put other children in their charge in danger.
Please don't think I'm having a pop at your child and obviously that's no help to you but I assume that's the reason.
I know when mine were that age quite a few parents used paid nannies to just do pick ups and drop offs, no idea how financially viable that is for you, but maybe being picked up as a group is part of the problem.