Controversial Issue-Teacher Unions & Tenure

Teacher Unions are there to help protect teachers rights and legal protections and all that good stuff.  I think teacher unions are very important, especially in this day of age.  Tenure, on the other hand, is that a teacher can be fired from a job within the first 3 years of teaching, but after that, can only be fired if there is a major issue and a legit reason.  This may be okay, but what happens when there are teachers that may not be doing their jobs too well, but there is not a major reason to fire them?  This could definitely make conflicts arise.  So with tenure, there may need to be some reforms.