A salary of $75,000 for 9 months is equivalent to a salary of $100,000 for 12 months.
Whether or not $100,000/year is outrageous or not is a matter of personal judgement, but I imagine somebody making $40,000 a year at Costco or something isn’t likely to have much sympathy for a person getting the equivalent of $100,000 a year who ends up complaining and whining about the indignity of being forced to make minimal contributions to their medical and pension plans.
I’m not sure if the flyer is a good idea or not, but one thing they should have done is include the 12-month equivalent value of those salaries, and perhaps the actual monetary equivalent of the pension and health care costs that those school employees find so onerous.
Teachers also do other stuff during their summer time. A friend of my wife's does tutoring for extra pay during the summer.