I think non-Britons dont really understand very much about the "school fiction" genre so conversations about these books always get a bit muddled.
Complaining about how its illogical one school house would be filled with dickheads and bullies in a school story is like complaining how its illogical that someone would have a street gunfight in a western. This is very well trod material. Thousands of these books exist.
It makes sense for how houses are organized in Hogwarts due to how the world is presented - there were 4 founders and you have a magic hat to scan people's mind to send them to a house that best matches their abilities/desires. It's hard to question, because "it's magic" can explain it without going into details.
However, it is also true than in the continental Europe you will find dickheads and bullies everywhere, not strictly in a very specific class. If anything, you could make it so that dickheads and bullies are from another school (where you need a lot of money to get in). Or that they form an unofficial group within the school. This is how "school fiction" can operate and feel much closer to reality (which is important when you don't have magic and you need to fit within the conventional school education system of the Normal World™). Movies did exactly that, back when I was watching them, as well as comics.