"The pupil thinks there is too much homework in school."
Translation:Eleven synes, der er for mange lektier i skolen.
This. This is a problem in general. I just had another sentence in this same lesson, and I thought, "based on my previous knowledge the answer should be such-and-such," but I hovered over a word to confirm that I remembered correctly, and it told me to use a completely different construction, so I did and got the question wrong. Don't screw with people like that, Duolingo. I'm using you to learn BECAUSE I don't already know, so of course I'm going to trust you to tell me when something it not correct, and if you say on a given example to do x, I will do x even if it's irregular because I trust you. Don't betray my trust.
Why is ''Eleven tror at der er for mange lektier i skolen'' incorrect?
When talking about institutions in general, in English you don't use the article, you just say "We had biology in school today." If you said "the school", it would be a specific building.
In Danish that's different, you tend to use the articled form with institutions: "Vi havde biologi i skolen i dag."