I think part of the problem is that as foreign teachers, we have very little power to actually enforce discipline, and the students are very well aware of this.
On an aside, as teacher assistants, we are not actually responsible for class dicipline. Technically that is the job of the Korean English teacher. Not that saying this helps you at all or anything but it might make you feel better

My students are generally pretty good, but on the occasional off day, or when my coteachers aren't around to play bouncer, I use the same methods you've used... adding minutes to class time if the class is too loud. conversely, if the class is really good, they can earn an early finish (3 or 4 minutes at the most) which gives them something to work for.
I have misbehaving individuals stand up. I tend to pick on standing students, so not only do they have to stand, but they have to *gasp* answer questions more often too.
I don't threaten my middle school students with their homeroom teachers ('cause that implies that I don't have authority etc) but if a class or individual is really super bad, then I cheerfully rat them out later.
Some interesting discipline techniques I've read about but haven't ever tried:
- Making students sit in dead silence for 2 or 3 minutes. If they make a noise, start from beginning. Being quiet is agony for a lot of kids.
- Squirt gun! I trained my dog this way, and it worked really well. I imagine it would work in a classroom too. Especially in Jan/Feb.
- Punish class but not the troublesome individual. Get the students to crack down on the troublemakers. I see a few problems with this (teacher is unfair!), but some teachers swear by this.
- Make classes earn special event days by awarding class tokens. With x number of tokens, the class can get x reward (pizza day, movie day, whatever). Can also get classes to compete with each other -- the first class to reach 50 points wins the prize etc.
- Assigned seating. Make kids earn the privilege of sitting with their friends.
- Force your coteacher to be in class with you, and ask them to heft their batons in a very threatening manner whenever a student breathes without your express permission. Or at least ask him/her to pay more attention to what's going on in class.
yeah theres more, but my fingers hurt.
cheers!