Ohio Bill 5: Why do teachers need unions? Worker right laws are not good enough for teachers?

I don't understand why teachers need unions. "This may include the negotiation of wages, work rules, complaint procedures, rules governing hiring, firing and promotion of workers, benefits, workplace safety and policies" from wiki for labor union. Now, I don't understand why wages shouldn't be fair market value, say comparing those government school district teachers to private teachers. I have had many teacher that are not worth what they get paid. Work rules? What type of unfair work rules could a teacher get? We have worker rights? Firing? Why shouldn't we be able to fire someone who isn't willing or able to do their job? Again, we have worker rights. Why do teachers need more than individual contracts to get benefits and pay? And why shouldn't they be able to be fired like everyone else as long as it's fair to worker rights laws? EVERY teacher seems to have pretty good working conditions, so I'm not really sure what the problem is there? Alright, they may have dangerous students, but, should we unionize store employees because they may have dangerous clientele?