teachers who play favorites
teachers who are unwilling to work with a child with a known learning disability, for example won't provide written notes for a dyslexic kid
teachers who are bullies
teachers who don't like teaching or a burned out
teachers who don't cover the material, but expect students to know it anyway
teachers who are completely inflexible; in time, attitude, approach to students and material
teachers who cannot connect with students or parents
teachers who pick on students due to race, religion or income