It is not a red herring as unions (not just teachers unions) help workers obtain better pay, working conditions, benefits, etc. They are not perfect-nothing is.
How do you suggest teachers be held accountable? It cannot be solely based on student performance as a teacher cannot make 200 house calls per night to ensure students are studying rather than playing on Facebook. This is the job of parents. Everyone agrees teachers should be held accountable; but how? There is no easy solution to this.
Yes, there are deadbeat teachers that benefit from union protection. No one denies this. However, there is no evidence that it is anywhere near the levels that you likely believe it is.
The vast majority of charter schools perform no better than major public ones. The ones that do should continue to receive funding and the ones that don’t should be closed. That was the original intent of charter schools, yet the failing ones often continue to procure public funding.
I have no idea what you are referring to regarding bribery and such unless you are referring to teachers unions backing Democratic candidates, which is generally true.