School should definitely be without the 's'. In the phrase 'at school', the word school refers to the general institution. 'At school' actually means, 'at a place where children are educated', it does not refer to specific schools or the physical buildings.
Here are some examples to show you the difference:
'I hate being at school': I hate being at the place where I have to learn (no reference to the specific school)
I have worked at many schools: I have worked at many different specific schools
'John is happy at school' - John likes going to a place where he is educated
'John is happy at this school' - John is happy at this particular school, he may not be happy at a different one
This works the same with any institution, such as:
So to summarize you only use plural if you are referring to specific schools, so the specific buildings. In your sentence you are talking generally about the place where Japanese students learn.
By the way also it should be 'Japanese children / Japanese students' and not just 'Japanese' on its own
I hope this helps!