Hi,
I've only been here for 3 weeks and am having the same problem! haha It's not too bad when the co-teacher's are there which is nearly all the time, but I just had a lesson where the co-teacher mixed up here timetable and didn't turn up and it was really noisy. I'm thinking of devoting a lesson for both 1st and 2nd grades next week to writing up the class rules (kids get to decide and make up a poster) and then telling them what will happen if they break them, or alternatively what the reward will be if they are good. I read somewhere about a system someone uses where they hand out 2 or 3 monopoly dollars (or something similar) at the start of class and the students lose a dollar each time they're naughty. If they have anything left at the end of the month they can buy a reward from the teacher. I don't really feel like spending money on them after the last class though!
To be honest, I partly feel like it's the student's English lesson and a lot of them don't want to be there so if they're noisy then why should I care...I'm certainly not going to spent the whole class asking them to be quiet. But then I suppose it's up to me to at least introduce some rules and see what happens. I have a feeling they've become used to talking in Korean and being noisy with their Korean English teachers as they've had no native teacher for a couple of months. The time my co-teachers want me to take to do an activity is ridiculous as well, with a 5 minute activity in the early part of the class taking up the whole class in some instances as they drag it out and we don't get to go on to something that might be a little more fun. I get bored along with the students.
Well, anyway, that's a long response! I hope things go better for you, and I was heartened to hear that the teacher teaching here before me had similar problems as you always think it must be solely your problem.
Good luck!