"The school is next to the restaurant."
I would translate that as "the restaurant is next to the school." Sure, they mean the same thing, but how does Duolingo know you didn't mix up the words for restaurant and school? I think as a learning app it doesn't make sense to accept anything that's semantically the same. It should be more strict.
Symmantic? Symmetric + Syntactic? Or you meant Semantic (which usually concerns a single word or phrase, not an entire sentence.)?
Your example is "The restaurant is next to the school." That's a needless reversal of the English sentence provided. It is grammatically correct, but not the best translation.