The real world is harder than high school because of the subjective aspect.
Because people can speak with their dollars and decide whether or not they want you, specifically, to succeed, in private enterprise, they can decide not to hire you or to give your qualifications higher scrutiny, or even lowball you on the pay scale just because there's an aspect about you, albeit arbitrary, that they don't like.
Homework? Sometimes you get some. You rack your brains on an idea that your boss has asked for to improve the company. Then you give it to your boss, who gets a promotion on your idea. They don't owe you anything because anything done within the company's auspices is company property.
Sleep? If you end up working retail...on a 3 day stretch, you may end up with a 12p-9p shift then a 5a-2p shift with another 5a-2p the next day. That means for at least 3 days a week for the duration of your job, your body doesn't know what's up.
Deadlines? Try missing one in the real world. Try to miss a credit payment deadline, rent, utilities and see how forgiving they aren't.
Disgusting school lunch? If you struggle enough financially like some have, a lot of times it's no lunch.
And the weirdest part of all that, if you worked hard in school....is that with your good grades, many clubs and sports, prowess in activities, academic or athletic awards is that you will wonder if these are the goodies for all your hard work.
Why?
Because in the real world, someone has to decide they like you before you even care what you have done.
Having that happen, unless you are with a successful network of friends makes the real world tediously impossible to comprehend.