As for the times when the teachers and students interests do align, that's fine. There should be plenty of popular support for the policies in those cases with or without the unions.
The evidence on the effectiveness of smaller class sizes is, at best, ambiguous.Second, while 'more teachers' may be one reason, most teachers also want smaller class sizes because they enable them to more effectively teach and most of them also want the best for their students and it turns out that smaller class sizes are good for students.
Sure. Teachers unions lobby for extremely stringent guidelines as to how to fire an incompetent teacher. This can range from merely difficult in some states, to nearly impossible in others. Many school administrators simply give up because they don't have time to fight the unions tooth and nail to get rid of a bad teacher...so the teacher continues (not) teaching his students and the students are worse for it.Can you be more specific?
Additionally, teachers unions lobby for seniority-based policies such as "last in first out," so that if a school needs to lay off teachers due to budget cuts they have to lay off the most junior teachers first, regardless of which teachers are actually good at their jobs.
The biggest problem is the inability to get rid of bad teachers, and the inability to provide enough merit incentive to recruit lots of talented people into the profession in the first place. Obviously there are plenty of exceptions...but in general the teaching profession primarily recruits from the bottom third of college classes. In countries with successful education systems, they primarily come from the top third.Can you substantiate this claim? Which policies are the biggest impediment to education and how have teachers' unions prevented them from being eradicated?
Teachers unions have stood in the way of these policies by insisting on overly strict rules for getting rid of bad teachers, and for completely eschewing the concept of merit. Obviously this is problematic because it means that less competent people, on average, will be teaching students...but personally I think it's also problematic because teachers can provide a role model for students. If students see mediocrity rather than meritocracy in the professional culture of the adults they interact with every day, I don't think it's particularly surprising that the students adopt the same attitude themselves.
(And before anyone yells at me, of course there are good teachers and whatnot. I'm talking about on the whole.)