Seating plans should be the first thing every teacher learns to do. It is no excuse if a) you have forgotten to do one; b) you didn't realise you had to do one; or c) you assumed - wrongly - that happy children in self-selected seats will work harder. They will not. They will ignore your wishes with the ardour of fanatics. They will award each other medals for the most perfect attempts to do so.
Impose your design on the class from the start. The seating plan should be ready to go before you even meet the students. You are the architect of their education, their expectations and sometimes their ambitions. It isn't too much to ask that you be the arbitrator of where their asses go.
The most subtle benefit of the seating plan is that you have, tacitly, without beating your breast or roaring, created an unspoken assumption that you have the authority to direct the students. It is assertive without being aggressive. Consequently, implementing a seating plan is one of the most important actions you can take in the initial stages of your teaching relationship with a class. And it is one that you can't afford to overlook.
There will be an occasional rebel who resents the tyranny of your presumptions, who will proclaim: "I want to sit here", "Why can't I sit here?" or, finally, the Wormtongue trickery of "I'll behave and work hard if you put me next to Becca". They won't, and if they have just told you that they need to be next to their compadres in order to be good, then you can be sure that they intend to be high maintenance.
So get the tone right from the start. If you let the students decide where to sit, what have you implied? That you're the Grand Duke of Emotional Intelligence? That you "get" them? That you care more about them than those rotters next door with their seating plans? No. You have told them that the classroom is a space for catching up with friends and that you don't mind if they chat. Don't be surprised if they start bringing in Kettle Chips and lighting pipes.
But how do you create a decent seating plan? Follow these tips and you won't go far wrong.
Front row seats
Keep your friends close and your cheeky monkeys closer. The ones that need most watching should to be nearest to you. This may seem unfair: most teachers would like to surround themselves with beautifully behaved, smiling children who cover their books in paper from John Lewis. But if you do this, congratulations, you have just recreated Mordor at the back of your room. Good luck rubbing all that graffiti off the desks. Maybe you can get the nice children at the front to turn around and keep order.
It's not a science
The seating plan is akin to a giant, 2D Rubik's cube. Every child you move displaces a dozen more, creating a hundred new geographical relationships. Do not imagine that a perfect seating plan exists, one where every yin is surrounded by yangs, each particle of matter and antimatter kept safely apart. Children's personalities and behaviours change more frequently than their shoes. Friends fall out. Alliances are formed and broken as though each school week is an episode of Game of Thrones. All you can hope for is the best fit.
There's a difference between the classroom being a social space (as any space occupied by a group inevitably is) and it being a space for socialising. Catching up should happen at break times. Lessons are for learning, so make sure that known friends and accomplices are parted.
Mix up genders
Another fairly safe way to minimise the risk of students going off task is to mix up genders. The boygirl combo is a strategy so old you can find it inscribed on Grecian urns. And it usually works.
It is tempting to put certain children together in the hope that the good conduct of one will rub off on the other. And in a class with a bell curve of ability and behaviour, it is inevitable that you will end up mixing those at different ends of the curve - unless you're aiming for ghettos of class, ability and maturity. But be careful of too much social engineering. Putting the nicest child in the class next to the most difficult is a strategy that often appeals to the teacher but is frequently far less attractive to the nice child in question. And don't forget that bad behaviour can rub off on to the well-behaved, not just the other way around.
Stick to your guns
Once you establish a seating plan, only you can change it. Never give in to pressure from students. Write the plan down in your book and photocopy it or display it on the wall. Make the children aware that you know where they should be. You will find that on Day 2 a number of students will "forget" where they sit. If you forget, too, then you may as well not have bothered, because the most ambitious and least obedient will rework your design in a heartbeat. Make sure that you continue to police your seating plan. The instant that absolute parity of rights is lost and someone gets to sit next to their pal, the whole group will notice and spontaneously combust from sheer fury.
Stuck for ideas? Use this handy spreadsheet to randomly generate seating plans.
See Tom Bennett explain why seating plans are so important in this TES Connect video.