First, lest start with elementary. Grades one and two are great, they will take the most energy from you out of nearly any grade. They lack English experience, unless of course their parents send them to English school after the day is over. I have ran into a few and when a second grader shouts 'excellent' and tosses you a thumbs up... It kinda throws you for a loop. Overall, they are super happy and excited to learn English.
Grades three through five are more or less the same. The only difference is that they have a better English ability. They require only slightly less energy (although that isn't saying much).
Rounding out the Elementary school is the Sixth Grade... This has been the hardest to teach. Its strange and you will see why in a minute. They are at the end of the road in Elementary school and most of them generally feel 'too cool' or 'too old' to learn. They are usually much more quite and reserved. Luckily, they are still 'easy' to get to co operate. Essentially, if you are willing to be like 'Ace Ventura' infront of class, it makes learning a lot easier.
Let's hop to Middle School. Remember the sixth graders in Elementary? Guess what? They are now the easiest group of kids to teach. Something must happen and trying to pinpoint why they are easier to teach is beyond me at the moment.
Second years are just about as easy to teach. They might have their moments but more or less they are real easy to teach. They will have a bit more language ability than the first years. Easier to communicate with and overall still fun.
Lets talk about the third years... Hardest group out of any of the above listed to teach. Essentially they are ninth graders in the USA. Which is strange, in the US, a ninth grader is probably more easy to teach than a twelfth grader. Alas, sometimes it feels like I am talking to a wall.
This is where that group think dynamic I talked about a while ago kicks in. You could ask an entire group of kids 'is my skin green' and no one would answer. They don't want to stand out if they are wrong or IF THEY ARE RIGHT. It makes no sense to me.
That is Japan however. Maybe more on this later.