I think you are on the correct path to attend to your health first and delay the work. What do you teach, and why does it involve standing for 8 hours? I'm a teacher, too, and while a rolling stool might be helpful, I'm wondering about a wheelchair for you? We have had several teachers in our building who have had to use one for a period of time, and one who is permanently in one. Students seem to rise to the occasion and become more helpful, understanding, etc. Others on this site have mentioned that the sight of them unloading a cane or walker from their car tells others to be careful around them--not to knock them over on a crowded street, etc. A rolling stool doesn't send the same message. The right wheelchair also would allow you to raise the new knee a bit, maybe even balance some ice on your knee, whereas a stool is feet down all the time. I find that position challenging, and my knee swells if I spend too much time feet down (as when using my computer), just like when I spend too much time standing. For when you must stand, can you stand mostly in one area? I would suggest getting one of those cushy mats to stand on. I notice quite a big difference to my knee when on marble floors (mall walking) vs sidewalks (a little softer) vs asphalt (starting to feel more comfortable) vs a limed path or a running track (ahhh!)
Think about the biceps-building potential of using a wheelchair! Also, you wouldn't have to be in it every minute--you could let students know that you will be using it when you start to tire, and for going long distances quickly in your school. I think of those long hallways with jostling student bodies, and my new knee quakes.
So while I vote for a delay in your return to work, and maybe when you do, phasing in part-time at first, I also think you can get some equipment that may really help you be successful when you do go back.