Thanks everyone for all the replies so far.
I agree with the sentiment that technology shuoldn't be required in these situations, but the Head has asked and I'm obliged to comply.
The school's MIS is Arbor. I will ask if they offer any sort of reminder, but I'm not sure what form this would take which wouldn't require staff to either have the website open or an active Outlook session, neither of which are guaranteed, which is why I have been trying to use Windows popup messages.
I assumed there would be something readily used and free for this and similar scenarios, but apparently not!