With all due respect to you good gentlemen, to heck with the law. The last thing I wanted when I was learning, or will want when I next go and do a rating, is to find out that the instructor is only just within the 90 day rule. Surely basic common sense says that this is a low minimum and anybody flying regularly, and especially teaching, should be comfortably exceeding it as matter of good practice. If you're not, surely even instructors are allowed a few solo circuits from time to time.
G