[AlexClips]

I am no expert, but I believe in most cases it is simply offered as an option either as part of the physical education curriculum or as an extra-curricular club, depending on the school. However, there are many schools that don't have a Kendo club too.

It starts from primary school, i.e. age 6 and up, and the content of the keiko varies largely on the school itself.

I have only practiced at 3 high schools (age 15-18) in Japan, two 'serious' ones (Kyushu) and one less serious one (Osaka). So I am don't really have an accurate view of the whole picture, nor am I involved in the Highschool Kendo 'circuit'.

Having said all of that, soon (next year I think?!) 'Budo' will be re-introduced as a compulsory subject for all students at junior high-school (12-15), and at many (maybe most?) schools Kendo will be an option. However, AFAIK they will still have to choose between Kendo and other Budo (Judo, Nagitana etc. - depends on the school)... This is one of the main reasons the Bokuto Ni Yoru Kihon Waza Keiko Ho are being developed as they are...