A unions mention 
I know you'll always marginalize it, but it is the main reason education in the US is declining. There is absolutely no need for it.
Good god
Give us your best reasons teachers need to be unionized that doesn't involve compensation.
Everyone should be unionized for the same reasons. Improved working conditions, protection from potentially being treated unfairly, and yes salary and benefit assurances are very important too. I know you won't care because it doesn't fit your conservative all unions suck agenda, but my wife has worked in four different districts, two with strong local unions, two without. The two with strong local unions, Wichita and Madrid, IA were much stronger districts with good teachers who care about their jobs. The two who didn't Cambridge, MA and Stratford, IA had much more issues with classroom management, class size, and turnover. The teachers were generally younger, not as skilled, or invested. If you are providing a job, any job, with a lower salary, worse working conditions, and worse benefits than other similar jobs in the area you are going to attract people not looking to make a difference but people needing to make money.