Sounds like same-ol-same-ol to me!I have been an ALT over here for quite some time and you will always find classes like this. First of all the HRT should be teaching the class - NOT you. You are the "ASSISTANT" as your job title already says. If you want to do a little more work, sure, why not.
Quite a few of the Japanese teachers are more or less forced to teach the English lessons - some like it, some don't. Bu that should not be your problem!
Tell the students that if they do their best during the lesson there will also be games. NO learning (=pain) - NO fun (=gain). Split the students in groups of 3 or 4 when doing games, they are a little more "controllable" then.
Try to convince the teachers to give out awards, like little stickers that you (or they) can produce on the computer.
Also you should be strict (and / or pissed of) when necessary. Tell the students "Fun is over and this is no joke". Many Japanese students nowadays are spoiled by their parents, grandparents and so on, meaning, they are not taught manners or respect at home. It's a "laissez faire" (hope the spelling is correct) in those cases. However, students should know where the limits are and how far they can go.
If there is anything else I can help you with, please get with me: klausdorth(at)web.de.
An 18-years-veteran (???)