I would say math, technical writing, and economics.
In math, the critical things that most kids (at least in America) get a chance to learn about at some point in school are arithmetic, algebra, and probability -- and they do need to be learned in that order. I see the end results of failing to know/understand math every time I go to a casino, as people ages 21 and up throw their money away in pursuit of get-rich-quick schemes like betting systems at the roulette table and prayers for godlike luck on Buffalo slot machines. They can't tell why 6-to-5 blackjack is so much worse than 3-to-2, or why the Big [Six] Wheel is such a ripoff. Then they wonder why they just lost their money so fast, and I have no interest in being bored to death by their sob stories.
Technical writing is a critical part of communication skills, and too many kids fail to realize that at some point in life, you are probably going to have to put together an e-mail or a letter that flows coherently in order to get your point across to another person. Leetspeak is not going to cut it in a lot of places, and it goes well beyond professional workplaces. Imagine that someone is at home, having problems with their Internet/phone service. There's a big difference between being able to explain what is wrong and what you already tried to do to a customer service representative, and sending a raging e-mail that says "My In73rn37z suXX0rs -- WTF Y U NO HAZ GUD IN73RN37Z ?????"
Now economics, not all kids get the opportunity to take that class before the end of high school, but the ones who do and say it's useless, are also more likely to be the ones who complain about simple things that economics could explain. For instance, how the price of gasoline or groceries keeps rising -- the ones who thought economics was useless might just say the entire system is rigged against them, whereas those who learned about supply and demand would understand that supply and demand dictates the market price.
So that's my take on what kids were wrong about when they said that they didn't need to know about certain things.