In my situation, by far the most challenging aspect of JET is the elementary schools. Teaching at elementary school is challenging in a completely different way than junior or senior high school, and is one of the reasons I think that ALTs at senior high schools tend to have a better experience (although teaching fewer classes with fewer students, teaching shorter hours, and teaching at fewer schools may have something to do with it).
There are a few things that make teaching at elementary challenging. First, you tend to see specific classes at irregular intervals. In Kamioka I see the elementary 2nd graders three times a school year, and see the 3rd through 6th graders 8-10 times in the same timeframe (I see the first years too rarely to really establish a pattern, but probably less than three times in a year). This means I see most classes once a month for a 45 minute period.
Next, you see a lot of different classes. In my situation1 I see about fourteen different classes ranging from 30 to 50 students per class. Keeping 30-50 students with widely varying skill levels engaged in your lessons can be a bit challenging. That is just a lot of students. In effect this usually means that more complex lessons are difficult to pull off, simply due to the number of kids in each class.
After grumbling about it for a while, its probably a good time to point out that the elementary lessons can go well, and can be pretty fun (at least afterwards). With that in mind, here is the sum of my collective wisdom on this topic after teaching around 150 hours of elementary lessons2.
Your first day of teaching a particular homeroom you will want to have a short introduction planned, but its important to emphasize the word short. The students do care about you, and are very interested, but they don't understand very much English, and even if your introduction was in Japanese, they are young kids, they don't sit and listen well for long periods of speaking (I mean, most college students have trouble with it, why expect it from eight year olds?).
I have developed a underwhelmingly simple formula for calculating the length of your introduction speech. 5 minutes + ( 1 minute * number of interesting props). (Multiple by 1.5 for introductions to junior high school students, multiple by 1.25 for senior high school students).
Do not go over the timelimit calculated by this formula. Shorten. Simplify. Add more props.
Also have an extra lesson prepared and by your side. You will inevitably realize while a smiling student walks you to their classroom that they are not in class 3-2 like your schedule says, but are instead in class 5-1, and that you just taught them yesterday.
The only tactic for surviving this situation in style is to have extra lessons prepared and on your person at all times when in hostile elementary school environments. This emergency lesson can be something simple. If the kids haven't reviewed in a few months, then sometimes I will break out a huge game of karuta3 and have the kids play karuta with the vocabulary from 3-4 different lessons. Mix that with some other simple activities (board races to add or subtract numbers you say out loud is another easy low-prep backup activity), and you can pack a relatively useful lesson into any classes that ambush you.
Planning elementary lessons is both simple and occasionally overwhelming. In my situation the overwhelming part is the complete absence of any guidelines or curriculum, but in the end that isn't necessarily a bad thing. Here is a pretty basic lesson format that I have been using to fairly decent result:
Introduce new vocabulary.
Activity using new vocabulary.
Review this lesson's vocabulary, or introduce next lesson's vocabulary quickly.
Many ALTs like to use songs and singing in their classes, but I, personally, have not. Those who do tend to insert the singing between the Review and Introduce new vocabulary segments.
I find that reviewing previous vocabulary for a few minutes each lesson is really essential for retention of the material. Otherwise the students simply don't retain it. Even if your lessons are amazing, if you don't review the material regularly, then the students won't benefit from them4.
Selecting vocabulary and activities is a bit daunting at first, especially when you are doing it completely on your own, but it gets easier after a while. The vocabulary can be almost anything, but its most helpful if you focus on vocabulary you think will help the kids once they hit junior high school.
I have done lessons on feelings (hot, cold, happy, sad, thirsty, hungry), sports (soccer, basketball, vollyball, baseball, badminton, swimming), classroom things (pen, pencil, teacher, student, book, desk), body parts (head, shoulder, leg, knee, foot, hand), parts of the face (face, eyes, ears, nose, mouth, eye brow), and less tangible topics like classroom commands ("stand up", "sit down", "be quite", "listen", "wait", "stop", "go"). Keep the vocabulary easy, and limit each lesson to 6-7 new pieces of vocabulary5.
The kids in elementary school cannot read or write English characters. This is your starting rule, and you should deviate from it if and only if you see definite proof to the contrary. For example the majority of my sixth year students can read and write basic letters fairly well (although many of them need a model they can copy from), as can many of the fifth years. Most third years will be hesitant to write even a single letter though.
I like to make connect-the-dots worksheets with the alphabet, and they do okay starting with the third year students, but that is with large pieces of the alphabet written up on the board, the teacher walking around singing the alphabet song, and the students helping each other a lot (and might take 10-15 minutes for many students to complete). At the other end, the majority of sixth graders can knock one of them out in three to five minutes, although some students will still struggle to begin.
And that is the majority of my accumulated wisdom. I don't really care a lot for teaching at elementary school, since my schedule changes so often without being told, the teachers don't always come to class, and its hard to establish a bond with the kids I see so infrequently. If you really want to become an effective elementary school teacher, you have to start by establishing relationships with the homeroom teachers. This means you need to hunt down the teachers and get them to listen your lesson plans.
By default the homeroom teachers, who you are nominally assisting, won't play any role in your lesson unless you seek them out and ask them to. It isn't necessarily the case that they don't want to participate, its just that they are as bewildered as the ALTs are, and don't know what they should do. From the ALTs perspective this is frustrating since the homeroom teachers know the students, know what they have learned, and are supposed to be taking the lead role in the lesson planning and teaching, but for the most part it just doesn't work out that way.
Your role at the elementary school may be a bit closer to gamemaster than you are comfortable with. I'm not particularly comfortable with it myself, but it is the expectation from the teachers that you will cheer the kids up and give them something to be happy about. It turns out that is also the expectation from the junior high school teachers, who will state pretty clearly in conferences that they aren't concerned with you teaching the elementary kids English, they are concerned about you getting the kids to their classes with a good attitude about English.
How you react to the expectations and situation is really up to you. I have tried to work a slightly more academic tinge into my 5th and 6th grade classes, and think that is has gone okay, but I'm not certain if it has really benefitted the students either. My personal goal was to get the students who really hate English to learn to read and write the characters. The kids who hate English in elementary and junior high school hate English because they can't read and write the letters and are falling further and further behind the rest of their classmates. I don't think that you can give these kids a good attitude by playing tic-tac-toe with English vocabulary.
But, apparently, the system disagrees with me on that one, and while I am leaving Japan in five months, I'm much less certain that the system will ever perish.