I remember living in New England when there was a teachers' strike and some of the letters in the paper said, teachers get the whole Summer off! So, some of it is just flat-out jealousy.
Plus, some Republicans such as Wisconsin governor Scott Walker have been successful at positioning themselves against unions for public employees and presumably this includes teachers' unions. In addition, conservatives have been largely successful at portraying the NEA (National Education Association) as a bastion of liberalism.
But, what if things were different?