Well, I imagine some of these teachers would be very unhappy if their students gave them fake notes from their parents.
Also, some of these teachers teach in schools that are giving what amount to fake diplomas, so I guess they don’t have a problem with fake doctor notes.
Your comment reminded me of something that happened in NY City. Back in 2007, Brenna Stewart, the daughter of convicted lawyer Lynne Stewart, was suspected of falsifying documents to cover nine days of absence so she could attend her mother's hearings. Brenna Stewart was a physical education teacher at the time in NY. Her mother was one of lawyers for terrorist Sheik Rahman, and was convicted of smuggling messages for him from prison.
Here is a link to the findings in the investigation. The recommendation was that she be terminated and made ineligible for any future jobs with the Dept. of Education in NY: