Appendix A [1] contains the salary schedule for Chicago Public Schools, which shows that in the current school year a first-year teacher ("step 1") with a bachelor's degree ("lane 1") earns between $57,000 and $69,000 per year, depending on the length of the school year in the particular school they are appointed to. A master's degree ("lane 2") bumps the pay by somewhere in the range of $4,000-5,000.
Being a coach on the sports team earns an additional $26.21 per hour of coaching, participating in after-school non-coaching, non-instructional activities earns $40.90 per hour, and after-school instructional activities earn $47.62 per hour.
It appears as though a teacher could add $5,000 or more per year with only an hour of extra time per school day.
If a teacher wanted to be frugal and save money, it surely appears possible.