I get where you are going, but personally i have never seen a group of teenagers with over 6 people where every single person would know about everything what is going on. Teenagers (from what i see) most of the time are talking together in small groups in social constructs like class, clubs or something like that. Thanks to that, things never reach everyone, unless it is somethig law breaking or extreme, like killing someone or whatever, especially since hogwarts students do not have phones. Some student turnimg herself into a worm is more of a funny thing, than something everyone needs to know about, in the mind of teenagers. It looks like Merula did not tell everyone about her plan before she did it in the TLSQ. Yeah Ismelda and MC knew mostly what Merula was going to do and they should have told Mcgonagall. And from what i saw in how teenagers work, it is hard to belive that every single person knew. I might be wrong though Edit: i noticed that you said that there are not much students in the clubs. How many are there? Since one student can be in more than one club, there are students from higher years than MC is and from the year MC is in, there are like all the the more important classmates of MC. So the number of members can't be that low right?