..... and meet feral barely house-trained creatures.
This happens to everyone. It's happened to me more than once.
What do you do after your heart sinks and you think "What the hell have I done?"
Well, first of all let me share a feeling. You've spent time, energy and sanity with this class so that they now sit at desks and the books, pens, furniture and whatever else is not nailed down remains in place and in the room. There is even learning going on! Just how good does it feel when you arrive at your classroom of now-tamed misbegots to a near riot? They've had a cover teacher, and it's so much better when it's a deputy head this happens to. But as soon as you open the door there is rapid movement and then silence and studying. Joy!
We all know the first rule is a seating plan. This is absolutely necessary for me as I have an appalling memory for names.
It doesn't take much to print out a class list of names, whether in Excel or just an ordinary list you can draw lines on. Make a table. You need 3 (or more) columns for each lesson for the week. For each lesson, you label a column Behaviour, Effort, Attitude (or whatever aspects of behaviour are particularly troubling). I separated out these 3 specifically as I wanted to separate effort from behaviour and target the rudeness and back-chat from otherwise well-behaved children. None of these have anything to do with achievement.
How does it work? First of all the class has to know. They also have to know that these records can and will be shared. For the final 5 minutes of the lesson you award marks out of ten for each category - openly. When the children are used to this they will suggest their own scores - and unbelievably own up to minor misbehaviours you didn't notice at the time. A triad of perfect 10s gets a gold star.
It's extraordinary how gold stars work. The children will compete. They'll collect them inside the back cover of their exercise book (AS LONG as they thought of it THEMSELVES!).
A week of perfect 10s gets a merit certificate that they can take home. Ignore the child who grudgingly takes it as though it's an embarrassment - so not cool, you'll hear from their parents just how proud they actually are of it. Especially if they normally go home with detention letters.
This works very quickly. Behaviour is individualised, assessed each and every lesson. Good behaviour is rewarded at once (a star) and continued good behaviour rewarded (certificates). The Greys (those kids you normally overlook) are included. And you've got a nice little record to spot recurring patterns.
For example, I had a feral class where I spotted that if a particular child, a Grey who normally scored highly on the table, was present, certain others' behaviour became abominable. When he was absent, they were perfect 10s. He was a catalyst, he was a Grey and not in the particular friendship group, he didn't do anything overtly to cause problems. Closer investigation showed that in fact Machiavelli would have been proud of him and he'd slipped under the radar for quite some time.
I have used a similar technique with individual students who require help to manage their own behaviour. In these instances I get them to self-assess every 10, 15 or 20 minutes, whichever is appropriate. This is very helpful for ADHD children and I shared this technique with my own child's teachers.
At the end of each time interval, you give the child a signal and they record how well they have been doing - whether behaviour, concentration, work done, or whatever you are focusing on. At the end of the lesson you talk about it. The child 'owns' his behaviour and his work effort. Stars and certificates (as well as the 'well done's) reward the desired behaviour. Again, it works quickly.
For those struggling to tame children who are immune to shouting and punishment, who know only disapproval, I hope this helps.
I also highly recommend the books by Bill Rogers.