I can understand...
I'm a middle school teacher and can understqand exactly what you're going through. We get a "planning time" that seems to end up being used for anything else, but planning.

Maybe my experiences can help too.
First of all, having a system you can fully trust is SO important. Personally, I use EssentialPIM for my calendar and lists. I print them out each weekend during a weekly review and use the printed copy during the week. This way I can write things on them and update them easily. I update the electronic copy during the next weekly review (if not sooner) If something comes up, like a new scheduled meeting, etc. then write it down immediately. Then it's scheduled and you can stop worrying about it. Same with your action lists. Even if it's during class, when I get a thought that needs to be done, I'll walk over to my desk and write it on my action lists right then. I also have a stack of index cards at the front of the room so I can write it there if I can't get to my desk. At the end of the day, I gather any cards I've written and update my action lists before going home. It will only take a couple of minutes and has saved a lot mental stress.
You're right, people will try to come to your room and take up your time. Depending on how your school operates or specific rules, you could lock your door and answer email or make phone calls. (This is easier if they can't see you through the door.

) Maybe you could take things to the school library and work on them there in a quiet corner. Updating lists, using a computer to answer email, and other tasks like that could be easier if you went to a more secluded location. Try to find "hiding spots" for when you really need to focus on a task (I'm not saying be unavailable all the time, but for when you really have to get out of the "public eye" for a while.)
Definitely do the weekly updates. For me, I include it as part of my weekly lesson plans. Update my lists/calendar and plan for the next week's lessons. They go together really well.
I even keep a digital recorder with me in the car for those thoughts that happen while driving.