The irony totally missed by that poster.
Utterly pointless to wear them then.
The classes being off doesn’t mean everyone in those classes has caught Covid you know and despite the precaution, may thankfully have not passed it on to the adult staff or each other.
Wearing masks can help prevent the spread for those who contract it outside of schools-you know, the ones caught it having play dates, mixing with others etc. without masks.
Doesn’t mean masks are useless. 
On the contrary, they are a good measure of protection. Maybe you would have preferred the whole school to close instead?
Of course, we don’t not wear seatbelts, despite accidents and injuries still happening..
The single issue of Covid is a pretty big one and it’s odd to berate posters for listening to the experts on that one. Especially with posters like you wanting to mix no masks, with the Indian variant and everything else opening up much more fully. Your societal issues are going to take a bit of a battering, if more children are going to have to self isolate, along with their parent/carer.
The cognitive dissonance is strong with this one.