Pros
Somewhat flexible management, holidays offered are decent, job very easy to do (even higher level classes). Training generally gets you ready for classes (though textbooks are not always the best material). Most school directors I worked with were very accommodating and friendly.
Cons
Very little incentive to do better at your job (you get small increases in pay with each new contract, raises in income do not keep up with inflation/cost of living increases/consumption tax). You'll make bank your first year (if you've never lived in Japan) then you'll take a big cut the next years as health insurance and tax payments take their toll. Work-life balance can be difficult for some, working late in the day until very late, or if you're lucky very early in the morning until mid-evening. Office culture can be suffocating at times, if you fall out of favor with your school directors you'll have a bad time (for a year or more if you return to that school).