Literally how does that even happen? I cannot imagine a scenario in which someone can just forget to put on pants / a skirt when changing, walk across a school, sit down, and then only then realize that they were half naked the whole time. Like, somehow this is actually the number one most contrived fan service moment I've ever seen in any series. It makes zero sense.
Plus, why the hell did he even stick his hand there?? Is a page missing or something? I can't figure out anything that would have prompted him to do what he did. Like, the "skirt" fell down, easy to miss 'cause the panel is a Cheeto, but whatever. But then how did that lead him to sticking his hand in her underwear? There's no justification given! He just does it!
Honestly, I'm sort of suspecting that this manga might have been written by an LLM or something.