i wrote "They think they have it bad," in my essay but my teacher said it was too informal.
I take phrases like that apart to make them more illustrative using the good old dictionary or thesaurus.
They: who are they? towns people, my parents. I'm choosing The Students Think: (tons of options here) use a synonym for think: assume, assert, are under the impression, I'm choosing determined. They have it bad. - find another word for bad or better yet be specific.
These students are determined to believe they have more homework than did earlier generations.
I don't know about the context (you might have to change the sentence structure) but you could talk about how they are self absorbed in their own problems, meaning they don't consider that other people have their own/bigger problems to deal with.
You might also describe them as ignorant (to how good they have it) or sheltered (From real life), depending on the context of your essay, which would give you more to talk about - why are they ignorant? who sheltered them and why?