It's not just about teachers' jobs--although that is important. It's about keeping public schools from becoming public entities or corporations. Why would that be so bad? This is why.
"Privatization does not mean that you take a public institution and give it to some nice person. It means that you take a public institution and give it to an unaccountable tyranny. Public institutions have many side benefits. For example, they may purposely run a loss. They are not out for profit. They may purposely run at a loss because of the side benefits. For example if a public steel industry runs at a loss it is selling cheap steel to other industries, and that’s a good thing. Public institutions can have a counter cyclic property. That means that they can maintain employment in periods of recession, which increases demand, which helps us get out of recession. A private company can’t do that. In a recession, they throw out the work force. That’s the way you make money.” Noam Chomsky