To cover a really obvious point, masks can't be used 100% of the time when eating or drinking for example. Having to wear a mask pretty much 24/7 is also a bigger ask than just at school which is likely to lead to additional compliance issues. There still could be benefits, but if prisoners only wear them for a few hours while being potentially exposed to COVID the whole day this is not going to be particularly effective.
At least for these for those who remotely understand the point of masks, those normally worn (or even really the best options) are not actually 100% effective, they merely reduce the risk of getting COVID-19 in a particular situation. However, because of the nature of disease limiting the rate of transmission is a big deal and can have a dramatic impact on the number of cases of COVID-19 in an area or community.