So, if you ONLY look at a 6-hour day, you could be correct, but most teachers spend 10-12 hours per day doing things like:
Developing lesson plans
Correcting/grading papers
Developing IEP's for students that THEY have identified as having learning problems.
Conferencing with parents until late evenings
Maintaining their certifications with unpaid CE courses and hours.
and the list goes on....

so the $25,000/year teacher who spends 10 hours per day over 180 days at school (not to mention weekends doing planning) would make (according to your calculations) $13.89/hr, a $35,000/year teacher:$19.44/hr, a $45,000 teacher: $25.00/hr a $55 teacher:$30,55/hr.etc
Teachers pay is slow in rising and dependent upon their education degrees, so to have salaries rise $10,000 their educational needs would be at least Master's degree, and then additional hours usually in 10-hour segments. The education is NOT free, nor are the extra hour segments.
College credit hours are on an average $594 per college credit hour. Masters degrees require 30-64 credit hours for a Master's degree.
so: $594 X 30 hours= $17,820
for 40 hours= $23,800
for 64 hours=$38, 016

Because teachers cannot go to school full time to achieve their credit hours, those courses must be spread over the course of many years. To obtain 6 credit hours/year is a difficult, but manageable task. 30 hours takes 5 years, 40 hours takes 14 years, and 64 hours takes 22 years.

Teachers ARE NOT PART TIME WORKERS. If you work 8 hours/day and your work year is 250 days you work 2,000 hours. At a minimum a teacher is on-site for 10 hours/day but brings home work and works weekends, so the 12-hour day is common. 12 hours/day X 180 days=2160 hours.