If art is subjective, why did schools enforce the idea that there's "right" and "wrong" ways to interpret literature?

I'm not talking about objective things like grammar, vocabulary, or structuring. I'm mostly talking about more subjective things like theme and mood. I get for younger students, since they need to learn to analyze and interpret literature. But for higher level students this might become more or a problem, especially when reading old poems and scripts. Why do schools make it seem like there's only 1 right way to view a story?