13 year old students are the most difficult to manage, at least for me. They are so full of hormones, ha ha.
Ok, this is what I do and it doesn't go bad:
<ul class="anf_list">
<li>From the first day I try to be very clear and serious about how a class should be.
</li><li>Respect for the rest of people: no interrupting when others are talking, no fights in class, no insulting, no laughing at other's English, etc.
</li><li>Of course, mobile phones should be turned off or in silent mode, as mine is.
</li><li>I assign each one a seat and they are not allowed to change unless I say so.
</li><li>Reading and Listening time are silence time.
</li><li>The more they work, the more time we'll have for games (just at the end of class)
</li><li>The less they work, the more homework they'll do.
</li><li>If someone doesn't behave properly, I write an asterisk next to his name. This means that they have one week to improve their behaviour or I'll report to the Head of School and call their parents. (I almost never have to report to anyone, students usually change their behaviour, at least for one week)
</li><li>I give extra work to misbehaving students and don't allow them to play games. This is what works most for me. Students usually have enough with one punishment.
</li></ul>
I always make sure they understand all these things at the beginning of the course, during the first week or so. If you are clear and act according to what you say, they'll respect you more. At this age, students are prone to "claim justice", so don't allow any opportunity for any injustice, on your part or on any other's.
Well, that was just my opinion. I guess it depends on the type of school and students that you are teaching to.

Hi Magg,
It's funny but it turns out I also work in a "Language School" (Academia de Idiomas) in Spain! Ha ha. Believe me, I do understand what you are saying. I agree with you in everything you said above. Perhaps the only difference is that, in our school, if a student doesn't behave properly he or she is "invited" to go away. Actually, that has only happened a couple of times and many years ago. It is required that parents are involved in the development of the child, who is reported to his/her parents if the behaviour is not proper. We care a lot about our students and they KNOW that their parents will be called if they misbehave.
Obviously, when you say that these classes are not compulsory and that you must measure what you say in front of them, is a great and sad truth. I think it's a matter of balance between making them work and making them happy and eager to come to class.

The biggest problem for me is the teenager groups, who usually are not eager for ANYTHING that means studying. What I do is make them work hard but, at the same time, make the class amusing, some humour is helpful once in a while. I sometimes let them talk about their school and teachers, and I try to understand their problems (after all, I've also been a student myself). They like to complain about school a lot, it's one of the things I've found that makes them happy.

Ok, when you say that you punish your students and they change their behaviour only until you are in a good mood again, it kind of rings a bell. It's sad to say this but, then, they may only behave if you are in a bad mood. Of course, this is not good for your students and for your mental health. The only thing that I can tell you is that in those cases you could manage your classes in a very serious mode, I mean, being very strict and clear about what you expect of them, not accepting that they make any comments about you or how you dress, and remarking that you expect good manners from them just as you behave with them. I make a lot of emphasis on respect for each other (today was my first day and I already told them that). When they ask me a personal question, I usually give an exagerated answer (a lie), for example: "Have you got a boyfriend?" (Me) "Oh, yeah, I have loads. In fact I have ten, each in a different country. And I also have ten houses, each for every boyfriend" You know, it makes them laugh and they know that I won't be answering any question I don't want to answer.

About your last comment, I've thought once or twice about sending one student out. Unfortunately, that's the one thing I can't do because students pay for those classes, so they must receive them. I usually make the conflictive student sit down next to me and far from his/her friends and have an eye on him/her most of the time. A possible disadvantage of sending a student out could be their parents complaining about that. Another disadvantage is that the student may well prefer to be out of the classroom doing nothing, so he/she could even misbehave more often. So, if the case is hopeless, one last resort is sending them to another classroom to do some exercises or prepare a fun lesson where that student will not participate but, instead, will do some more exercises, while the rest is having fun (isolate him/her a little). I'm almost sure he/she will start behaving better after that.

The truth is that I spend many hours of the day thinking about strategies, visiting web pages, etc, to improve my classes. I'm not sure if I can post some helpful websites. If I can I'll give you a couple of adresses that may come in useful, ok? This is a constant struggle, haha.

Hope this helps a little. And sorry for this long answer.