Doesn't it feel like we've missed something? The protagonist keeps referring to preview events, e.g. something that happened during the entrance ceremony and the manga even shows flashbacks to some of them. Even the description says "hentai is back! ...", which indicates that there's been something before this chapter and that it is not the first time we've met these characters! p26

So my conclusion is that this isn't a oneshot. Instead I think it's a story that consist of at least two chapters, maybe three, and this is the second chapter. What's your opinion?

That's right, there is a previous chapter that wasn't translated. I found a link for the chinese translation if you are interested : http://www.dm5.com/manhua-biantaitianxin/
Indeed there are two chapters 🙂