It is one of the most tedious and ridiculously over-tasked and underpaid jobs in Japan. The workload is ridiculous. The above-mentioned six day week doesn't nearly cover all the extra-curricular activities and festivals and culture days and prefectural athletic meets and late-night test-correcting, and teacher training sessions, and weekends spent away chaperoning kids during field trips, etc... Plus the general belief that teachers should be full-time guidance counselors for all aspects of a kid's development makes it very difficult to have any kind of anonymity in your life. Plus the almost soul-crushing hierarchies in most school systems...yikes. It doesn't surprise me at all that there is a shortage of teachers.
In the past (as in, before the 90s) it was seen as kind of a cushy job, because the rest of the country was working just as hard, including weekends, but the teachers at least got some time off in Summer and year end, and it was more or less guaranteed lifetime employment with decent retirement benefits. But now its just drudgery unless you have a deep love for it.