Management doesn't even think they should be allowed to go for a piss. All lesson time is sacred, even if the kid is getting nothing out of it and spoiling the environment for everyone else.
Mostly the only place you can send them is whatever the school calls the naughty room. Some schools have a 'sensory space' where kids, particularly ASD kids, can go to get some peace or do something they find calming but this is not nearly common enough.
You can also have a school dog, which is a great way to trick troubled kids into going for a walk outside and opening up to a sympathetic adult.