This explanation is based on an as yet unexplained situation. Why does she have her bra on?That's because she didn't wear a bra to begin with. You can't put back something you didn't bring.
I agree with hzjujk explanation entirely, basically exactly what i would have said on the matter.
She put her bra on to see what? It would have been normal for her to pose in front of the mirror without a bra on to see what she looks like to those watching her. Sophia knows she won't wear it during class, right?
Pose with that bra on in front of the mirror for what?