Thanks for the responses. Have another class with them tomorrow and will try to implement some of the advice given.

I've taught mostly kids classes for the last 2+ years and I never had any training so had to learn on the job. I remember how difficult it was with at the beginning to control the class and keep them interested but now I feel I have the right formula and it's so much easier. It's so true as someone said that you have to "win the crowd". Teaching a big class of teenagers is just such a different dynamic compared to teaching children so I just think it will take a bit of time to get the format right.

One to one is of course the easiest, but although teaching a class is harder I find the time usually goes a lot more quickly so in many ways I prefer it.

Regarding training (TEFL, CELTA etc.) has anyone found that to be very useful? I never bothered with it because I feel it's much better to just learn on the job and figure things out for myself. But I've had plenty of moments in class when I feel like I don't know what the hell I'm doing and a little training would have come in handy.

I like one on two, because it requires the students to listen and understand as third-person participants, which is an important language ability.

I there's a big problem in China today regarding teaching and learning. I've notice that 'most' students lack the 'hunger' of learning. They lack the 'yearning' to learn. Why? Too many students spend a lot of time on the internet playing those online games. Too many students are spoiled by their parents to the point of feeling 'entitled' and lose the 'eye of the tiger' feeling. While it's true that in China students are given too much homework, the other reasons are because more of the reason for the lack of participation. They just don't give a 'shit' about learning. They are counting on their parents to support them and baby them. 　

@liumingke: I also think one can blame the reduction of education in China, as well as in plenty of other places, to the idea that it's only about cramming information, and that the goal is simply to soak up 'correct' answers, almost all of which are proclaimed to be fixed. I.e., I think there's a lot that could be better with educational policies in China, as well as official attitudes concerning true/false, right/wrong etc., thought in China generally.
In other words, it all becomes boring.

It is important to understand how old the students are, and their place on the education treadmill. That way we can give you more useful info.
There is a huge difference (a continuum of differences) between 13 and 19 year olds. Those who are approaching Zhong Kao, or Gao Kao are different to those who have already been through the Gao Kao mill, and those refugees from Gao Kao system also have different motivations.
These differences are not only in attitude but also in age appropriate teaching materials. Please advise.

Regarding training. This is not essential, but I have found it to be worthwhile from personal experience. I had been teaching for 6 years before I did my CELTA. I learned from it. The most useful things I learned were why some of the things I had done worked and other did not.
Looking back, if I had done CELTA in the beginning it would have given me a better foundation and I would have been a better teacher earlier on. Additionally, having a CELTA did open doors to other teaching jobs that were better paid and more rewarding, and this in turn lead to positions that were better paid. Eventually, leading on to subject teaching positions, as a subject teacher with CELTA is more marketable and ultimately more employable.

These kids are 18, and actually they're preparing to go to university in Thailand. But they have to pass a test for English and Thai so they still have to study very hard - they're in school 6 days a week 9am to 10pm. A tragic waste of life.

As has already been suggested, I strongly recommend getting some teaching materials. We all have our favourites, and I can recommend the 'Interchange' series of books. I have used level 3 with university freshmen English majors.
Every activity in these books can be turned into a communicative activity. Quizzes, vocab, and even listening exercises can be made pair/group discussion activities (important only one person in the group can have a pen, if you have 4 SS and 4 pens, you no longer have a group but four individuals working at the same table). You just need to encourage them to use English in discussion, but expect them to slip, and some students will not have the ability to do this so easily, avoid making a big issue of this as you can't win.
All dialogues can be practised using variations of the wheel (inner circle faces out, outer circle faces in to a partner, and rotate in opposite directions frequently to change partners) so all SS get to practice will other classmates and not just their buddy. It also gets them up and moving. This also allows you to do any prep for the next exercise.
The biggest issue will always be classroom management. But keep changing what you and they are doing, if you can move furniture around, and certainly get them out of their seats.
A note on games. These can work or fail terribly. Penny Ure wrote a book of 5 minute language games. However, students here are not used to game play, and even with adults it can take 30 minutes just to explain how a game is played.
Remember to scaffold all activities. You will need to spoon feed SS the answers in the beginning, they have come out of a system that does not encourage or reward independent thinking, this is a skill you will have to build in your classes. Using a book like 'Interchange' does this scaffolding for you.
Notes on homework. Ha! Good luck with this one. As you have already noted, they are working long hours. If you have been told you must give them homework, then give homework that suits you and suits the SS. They will cheat, because they don't have time. Don't make a big thing of it. Make homework that is easy for you to mark, and easy for them to complete (honestly or otherwise). Giving them a 500 word essay is tough for them and tough for you to mark. Only do it if required by the curriculum. If they find you class easy, they will put up fewer barriers to learning. Remember that some students will 'rebel through mediocrity'.

Note on you the teacher. What is your role. Usually the foriegn teacher is there to augment the SS experience. The real teaching/test prep is usually done by a Chinese English teacher. Remember that you are not a parent, and easy trap to fall into. If try to be a parent you will just become an instant a-hole. The SS will then withdraw. And watch out for the classroom stool pigeon. Most classes have one.

i was at a college att to a uni. expectations of teachers were low. basically turn up sober, keep the studnts happy and give them all a good pass at the end of term exam. there was no expectants of teaching them

My advice is you may need a CELTA to get the better jobs, work in the middle east (where the money is) or to obtain a visa in some places like Vietnam, Indonesia etc.
Think of the CELTA as an expensive, work intensive way of opening doors for yourself. Once you've got it take what you want from it and forget the rest. It's like driving a car - During the driving course you have to drive like an absolute spaz to conform to the standards laid out by the instructors. After you pass you can drive whatever way suits you best.

Teaching teenagers has to be easier and more fun that kids, surely? Give them some grammar, let them practice and have the aim being that by the end of the class they can add their existing vocab to the formation you've taught. Make sure you work towards the goal of letting them do something interesting with what you've taught them at the end of the class. You're not going to set the world alight playing videos every week or passing them on exams that may be beyond them.