That's fantastic... On the band! Try not to feel down cancer is scary to everyone most people are just happy - 1) it's not them - 2) you're fine so they can stop thinking about it. During my treatment and even after my diagnosis my sister in law mentioned my cancer a total of 3 times - she never once called even to see how I was doing or offer help (she did come by the hospital one day of the 10 I was in). In fact the day of my surgery (14 hrs) she asked my daughter to baby sit so she could go to a "ball" - I have no siblings - my mom is gone and at the time I was in the hospital my dad was in Europe for 3 mos! My inlaws - are great and so was my husband but even they are happy it's over with (at least in their eyes)
Most people don't get it.
As for your boss - union? You have one... I'm assuming - time off for illness and check ups should be built into your contract... And believe me I'm not a union peron per se - but it do know 2 things - school boards don't like principals with a lot of grievances - and the best way to shut someone like her down, is mention union.

Good luck - heal well!