Wearing a mask slows down the rate of aerosols emitting from the mouth.
It catches some cornora particles depending on the fabric, layers how it's worn.
More aerosols are emitted when shouting, or singing. Imagine someone doing that right in your face, neither wearing a mask.
Lots of strongly propelled aerosols flying into your face, eyes, nose and mouth.
A mask slows down these droplets, hopefully many would catch in the mask and as they do get through, massively slowed down before they hit the person your talking too and hopefully hit the ground before they float to someone's face.
The person wearing a mask, face to face with a non mask wearer, will still have someone's aerosols flying straight at them, into their eyes, landing directly on their mask and depending on the grade, into it.... Floating in the air... Because they have nothing to slow or divert the course of the aerosols
Having a window open can help with a breeze helping to scatter the droplets...
To be protected to a better degree I would expect to wear an fp mask with a visor.
But why does it matter, surely asking our dc esp those in secondary school, year 5 and 6 at primary to wear masks is the least we can do in this final push for a few weeks IF we want dc in school and teachers safe?