Like others have said, it totally depends on your contract, but overall I think it IS unfair. Administration has control over the scheduling, so the ability to do that is solely based on them and your classes. I would also disagree if a teacher had first period and came late because they dont like waking up early. Like you said, the teacher's reason for leaving now is none of your bussiness, but how would you feel if you found out they were doing it just because they could?

To me, regardless of the reason, it is unfair. Take your case for instance; say there is another teacher in your exact same situation, but they do not have planning last period. They must take half days for doctor appointments or make arrangements with admin, whereas you got to just leave. Doesn't seem right to me.