Our DD had all three of the flavors that HowlerKarma lists, though not so lucky in category 3. In addition, DD had two other categories:

4) Teachers that had too much going on in their lives to focus on teaching. DD had teachers with cancer and on chemo (multiple times), teachers experiencing a difficult pregnancy, teachers with new babies that were getting no sleep, and teachers with second jobs. While our family felt a great deal of empathy for their personal situations and anger at the district for not supporting the teachers adequately, we could not asses their ability to teach as almost no teaching occurred. (When the poor souls who were ill became too ill to attend, the district would fill in with week long substitutes for up to 2/3 of the school year, but that is a whole other topic. Suffice it to say the short term disability situation appears to be very poorly handled.)

5) Teachers that the district did not allow to teach. These were the teachers that taught specialized classes with classes at multiple schools. This meant, in our district, that the teacher would never be available for after school or between class help, could not sponsor any clubs, could not meet with students or parents, had to move all their supplies multiple times every day, had reduced access to textbooks, and had a very high load of non-teaching activities (the paperwork for each school, the staff meetings, the required training, etc). Picture a choir teacher that runs from school to school and has no school provided sheet music for the students and has no ability to schedule rehearsals outside of class; or a German teacher (not the language DD took) with no textbooks, put on the foreign language committee for 2 middle schools and 2 high schools, that could not put up posters and had to clean every board and remove every trace of their "borrowed" classroom after each class.