Mahiro, relax already. You're a girl. No one is going to shame you for wearing proper underwear.
It's actually kind of heartwarming that Miyo finally has someone she can do stuff like this with. She's been finding herself in that awkward position where her body is changing while all of her friends' aren't, which can be a very uncomfortable time for a kid going through puberty. Having Mahiro there now means that she's finally not alone in this.
In other news, I hope CoolOtomegane is ok.
@chilzhul: They're not saying it's too hot for winter, they're saying it (meaning the current weather in the series) is too hot for the winter
uniform. Which is a perfectly valid thing to say. Just like it can be too hot to wear a heavy winter coat, or conversely too cold to wear flipflops, shorts, and a tee-shirt.