They are paid a pittance by the district and treated like glorified babysitters by the parents.
I'd like to see some numbers here - please explain this "pittance" teachers are paid...
To become a teacher in most states (all I'm familiar with, my I have not looked into all) require little more than a BA degree to become a teacher.
Once hired, if the teacher can hold on to their job for two years in a union school district, they get "tenure", among other things, it is effectively job security for the rest of their lives.
Teachers have, typically, fairly generous health care benefits, as won though historical contract negotiations and labor disputes.
Teachers are off every school holiday (by definition), and yes, some take their work home and finish up grading papers at home, but if they tried a bit harder they could likely get most of that done if they stuck around school for an hour or less - they likely choose to go home and meet their kids coming home from their school.
Union teachers (not all teachers) are paid fairly competitively for the jobs they do, and typically know in advance their next three to five years annual salary increases due to labor contracts - that something few Americans have.
At retirement, teacher pensions are very generous and include continuation of their (previous) employer health care coverage for life.
If you really want to argue teachers are paid a "pittance" I really need you to show me your evidence.
If teachers are buying classroom supplies because the district won't, that's on the parents (AKA the taxpayers) who should step up and cover the expenses.
The next time a teacher says they are under paid, ask them what their salary is, if they won't tell you their salary, they aren't underpaid, they just want more.
If you want to claim teachers are underpaid, make sure you take a good look at their union contract first - see if they really are underpaid, their salaries are public record.