Who is 'oppressing' these teachers?
Administrators who suddenly decide to have a 3 hour meeting at the very end of the work day. Administrators who fire qualified teachers and hire their unqualified good buddy for the same position. Administrators who refuse to purchase enough text books for the number of students in a class. Administrators who don't plan man-power properly and have 40-50 kids in a classroom built to hold 30 max. Administrators who give performance reviews based on the attractiveness of a teacher. Administrators who maintain physical environments that are not condusive to learning (too hot, too cold, dirty, depressing, interruptions to class time). Administrators who assign extra duties that interfere with student's education, at no extra pay. Administrators who create a schedule that does not allow for even a lunch break, much less a restroom break for the teachers.
All of these examples are things that actually happend in the district that I worked for, and had clauses in the contract that were added, negotiated by the union and the school district.