1) You are assuming someone could get a comparable job for just 3 months (more likely, they'd be lucky to even get a minimum wage job for that length).
2) You assume that during those 3 months, teachers are on vacation just like the kids, and not doing any training, lesson plan improvements, etc.
3) You miss the GP's point that you are only taking days into account, if you count the hours teachers work, their hourly rate would be much lower as plenty of teachers (my wife is one) work 10+ hours each day, and exceed that numerous times throughout the year (extra grading to do, parent teacher conferences, etc...)
I do agree to some extent with you that there are many problems with the education system that are not salary related, but they do contribute to the problem.
I know one other person who's hours, and stress level are the same as my wife's, and he makes quite a lot more then her.