According to the link, this is based on a real event for Claire.

この前実家のクローゼットの奥から小学生の卒業式で履いた制服風スカートが出てきて好奇心で履いてみたんですが、若干お尻がきついようなもののウエストはいいかんじだったのでちょっとうきうきして、しかしながらこれってもしかしてヤバ行動なのでは…と冷静になりました。

"I found a uniform-like skirt I wore to my elementary school graduation ceremony in the back of a closet at my parents' house the other day, and out of curiosity, I tried it on. It was a little tight in the hips, but the waist felt nice, so I was a little excited. However, I calmed down and thought that this was probably a dangerous move."