Mine sit in ability groups for maths and literacy but apart from that can sit where they like ( although they do get quite possessive of their maths places!) If they misbehave, they move! If anyone is unhappy with where they are (usually the girls because there are too many boys on their table!) they can also move as long as there is space ( I have 4 spare seats in my class so shuffling round happens quite a bit!) It's quite interesting to see the shuffling around that goes on!