Hi Jules and welcome to the forum.
As your pullets are several weeks away from needing the nestboxes for laying, you could try blocking them off by putting large flower pots or boxes in them, anything that will fill the space so the girls can't get in. Then it's possible they will just roost on the floor of the coop, and will need to be helped to learn how to perch. It's likely they haven't been kept anywhere with perches yet, as many people believe it's best for growers not to perch until their bones are more fully developed. So if you go down after they have roosted and are sleepy, you could try lifting them gently on to the perches. They will soon get the idea, though you may have to do it for a few nights. One evening you'll go down and find them all roosting neatly in a row on a perch, and a few days after that, when you're sure they have all cottoned on, you can remove the pots so they can begin to explore the nestboxes as they come up to lay. It won't harm them to sleep in the boxes or on the floor of the coop, but it's best to encourage roosting on the perches as they will stay cleaner if not sitting in their own droppings. The wrong roosting habits can be hard to break once established, so a bit of trianing now will pay off in the long run. Perches should ideally be higher than the nestboxes as they will always want to perch as high as possible, so if yours are the same height you might need to do a bit more training than if the perches were higher.
