It'll give those boys so many more opportunities to humiliate girls (and those who don't identify as girls but biology has a habit of not caring about identities).
Oh, look, she's too poor for tampax!
Do you use washable pads because you're poor?
She's such a tramp, she keeps dirty towels in her bag.
Look! they're stained! Ugh! She doesn't even know how to wash them! She's too poor for soap!
Bet she uses an old sock because she's too poor.
Oh, look, she's going to get the poor people's pads! Doesn't your Mum love you enough to buy them for you?
I saw you coming out the chemist. Were you there because you've got your period and you're too poor to buy your own?
Boys already learn enough about periods to make girl's lives miserable at school if they want to. Even when taught about them separate to the girls and by male staff/visiting educators. And teachers already know the potential for disaster in attempting to teach mixed classes for this section of PSHEE. That's why they don't do it.
No amount of 'Let's be understanding of the uterus havers when they become menstruators, mm-kay? Here's our latest snazzy poster' from the council Period Bro is going to change that. The boys who are decent people will be decent, as many are, the ones who are immature twats will still be immature twats, just with extra ways to do it. The decent ones don't talk about it because they know it's not their place, they just help when they know they are needed (ie, finding supplies if a friend's been caught out, telling the arseholes to fuck off, that kind of thing).