Oh, I can so relate to this. My 10 1/2 y.o. oldest DD is giving me grief over this issue and she's NT. Right now she needs the coverage more than the support so I'm allowing her to layer a cami underneath her shirt. But I did warn her that probably in another 6 months or so she'll need to start wearing a real bra.

1 take her shopping with you for your bra. Ask if she would like one too.
2 explain that boys are different than girls
3 find her a similarly aged girl friend that she can talk to. There are penpal websites. Or even in another classroom.