I never got in trouble for reading novels in class above my desk (about half my classes from middle through high school.) Actually can't fathom how that's rude. Conversely classes being compulsory and such and being compelled to sit through repeated material at a ponderously slow rate seems rude. Seems better than falling asleep or doing something overtly disruptive. Or worse I would pay attention and get highly engaged in the material and ask overly complex questions to enrich my understanding; now that's disruptive.