1. Only really rich kids go to the private high schools with ridiculously high tuitions.
99.9% of the teenagers who attend a private high school in Manhattan, especially the Upper East Side, have overflowing bank accounts. They do not that are plenty of others who are starving every day and saving pennies in order to survive. To them, money is nothing, a normal after school activity would be going through the designer stores and boutiques and max out your credit card. The money goes to them as easily as anything, a couple thousand dollars to them is nothing. They don't work for what they get, they think that they should and that their parents ought to give them the money.
2. They don't really care about their grades because they don't have to.
In order to keep up the school's fantastic reputation and above average GPA, the teachers will go out of their way to achieve that. They need their students to all get into the Ivy Leagues, and since the students’ grades depend on what their teacher gives them,
the students have a rather easy academic life. Also, since there are so many occasions and entertainments that are not academically related, doing their homework the first thing they get home is not necessarily their everyday agenda.
3. The students are not necessarily "good" and "nice".
With their overflowing bank accounts, and taking limos with private divers every morning to school, they don’t really have to be good and nice to anyone to get things their way. They have powerful families to back them up whenever they need. With that much money, they don’t care if they spend a whole chunk of it on alcohol, buying drugs, or just completely wasting it. With their bank accounts, a Porsche is worth almost nothing. They don’t have to drive safely because if they totaled their current Porsche or Tesla because they were drunk, they can get another one without hesitation.
4. They don't have much respect for anyone.
Being used to getting everything their way and not having to worry about the consequences of their actions is what they grow up being used to. Since they are rich, they think of themselves as a higher class then other people who don't have as much money, they think they are better than others, and that rich or poor is what defines you. Most of the teenagers think that they are better than others because their closet is the size of other's living rooms, or that anyone who lives in Brooklyn does not deserve to be part of the elites.