The female teacher just barges into the boy's dressing room like that, eh?
Page 13 - Sensei blowing the whistle like that looks like something else entirely.
Look at her face!
I like to think everyone already knows those two are girls and are just playing along because it would be funnier this way.
I mean, there's absolutely no way you can hide those humungous breasts by doing a piggy back! XD