It's not as nice as you make it sound. They get the same holidays as any other government worker and while they technically get the summer off in the sense that they aren't in the classroom, they still have administrative work plus prep for the next year that they have to do. They don't pay into SS because they get a large amount of their checks deducted for their pensions (which is mandatory in most states). Some states they do pay into SS and get smaller pensions. They still pay medicare.
Secondly, jobs in yuppie school districts aren't just being passed out like candy along with 80k salaries. The starting salary for any teacher straight out of college (if they are lucky enough to get full time employment) will probably be pretty low in comparison to cost of living for the area. I also seriously doubt that yuppie schools have much turnover. Most of the jobs are in shitty districts where you have like 40 straight outta Compton kids packed into a classroom who's parents are probably more unruly than their children. The state & federal education funds are also constantly being fucked with, so they go long stretches without cost of living raises and market adjustments. Top it all off with the fact that you have to have at the very least a Bachelor's degree and a state license, and lets face it, unless you are willing to work in Compton you'll have to have a Masters, so you are guaranteed to have a nice student loan payment to go along with your low starting salary.
So yeah, they may not be under paid in a numbers to numbers comparison, but their bullshit to salary ratio is a lot higher than a lot of other jobs they could get that paid the same amount of money.