I find 3 hours is able the max time I would want to wear a single mask. I had to push it to 4 once and hated it. For a full day at work (or school) would look something like this -The issue would be wearing masks effectively. As already mentioned, once they get moist, protection is reduced significantly. And then there is the fun time of lunch. So maybe pack 2 masks and change after lunch? The other issue is that to put the mask on, you should be doing that with sterile hands... which can be difficult for kids.
-Mask 1 put on mask just before entering the building (or bus in my kids case).
- Mask 2 recess or am break. Wash hands, and switch masks
- Mask 3 wash hands for lunch, take off mask for lunch, wash hands again to put on new
Someone could wear the same mask and let it air out for periods of time and use only one or two masks in the day. I am planning for 3 a day with one extra just in case. Sterile hands is not difficult if the kids are taught to wash their hands and have a place to wash.