I did respond some time ago but in case my message didn't reach you I am replying again.
I like the idea of a 5 star rating system, 5 being for a good and highly recommended school and 1 for school teachers should pass up. I would add one more category: a black X as in Immigation should not issue Immigration Visas and warn foreign teachers about them, some black list equivalent.
In fact Takamatsu Immigration would do this with Lingo School if the teachers stayed long enough to file formal complaints, but most teachers just run away, leaving Immigration no evidence to work with. Anyway the black X would mean don't bother applying, they should be shut down.
Criteria could include: Does the school management offer an orientation period for new teacher? Does the management support the teachers? Do the students see that the school is working with the teacher or do the students feel they can get the teacher fired any time? Does the school try to accommodate their teachers by providing teaching aids, educational games, by solving problems flagged by the teachers? Do they help a new foreign teacher settle in? Do they help the foreign teacher learn Japanese and adjust to the Japanese environment? Do they pay on time and correctly? Are the teachers required to work on Sundays without pay? What type of performance assessment to they make? constructive? punitive?
I hope this helps.
R. Martin