2 part system. Laminated mat goes on you tables of 4 with table number/group name on in whiteboard marker and coloured arrows pointing to seats. Sheet is filled in for each class with seats assigned by table and colour. As classes arrive seats can them allocated quickly with pupils being given their table number and seat colour. When working in co-operative groups it is then easy to assign roles and tasks by colour. I've used this to good effect when teaching several classes in a day for non contact time in a room which was not permanently mine so all resources had to be removed at the end of the day. This system could easily be moved from room to room.