How Often Do Japanese Kids Go To Cram School?

Cram schools, or gakushu juku (学習塾) are not too far off from the sort of extra-curicular schools that students in other countries attend. Not all Japanese kids attend them -- about 1 in 5 do according to a 2011 study. But those that do, tend to have much more success both academically, and in subsequent careers. Indeed, having them presents such a clear advantage that any parent who is well heeled enough to afford one will insist that their kids attend.
Kids that do attend may not do so through their entire school lives. Their instruction comes in most handy during the run-up to entrance exams -- namely the infamous "examination hell," that panic-stricken time when middle school kids have to study like crazy for the entrance exams to get into their high school of choice. In that way, juku are similar to SAT prep courses from places like Kaplan and Princeton Review: they offer sample tests for well known area high schools, and will help kids study for those specific exams. (There's an "examination hell" period to get into college as well, but the one for high school is widely regarded to be worse.)
There are other sorts of juku: there are ones for remedial kids, or for who need some extra help in a subject or two. There are juku that specialize in topics that aren't normally covered in schools. (For example, there are Juku in American for Japanese students to keep their Japanese language studies up.) They are mostly for-profit institutions, some of which have franchises in multiple locations. They cost around ¥260,000 (around US$2,600) per year to attend.

And many kids don't seem to mind going. Actual, normal school time is spent primarily in mind-numbing lectures, with limited interactivity and the same classmates for the entire day. Juku allow them more focused instruction and individual attention from a teacher. Some attend the schools with friends, and come out of the lessons with a bit more confidence in their understanding of important subjects. And besides, it's not like they have much choice in whether they attend one.
With Japan's population in decline, getting into certain schools isn't quite as hard as it used to be. Be that as it may, juku are more popular than ever, as Japan's parents have become increasingly frustrated with an educational system that doesn't really go out of its way to help an individual who might be struggling. The benefits of having this additional schooling are so clear that their popularity is a constant embarrassment to the Ministry of Education: visible proof of the failures of the educational system. It also puts kids from lower-income families at a marked disadvantage.