In both cases I'd say "son difíciles de entender", and any other audacity is wrong.
Seriously, I don't know if this usage is accepted in other regions, but probably the teacher believes "difícil de entender" is an adverbial construction, and since adverbs don't agree in gender or number with the noun, "difícil de entender" would be correct.
However, "difícil" is
always an adjective and "de entender", here, is a complement to this adjective. "Difícil" is not a word like "rápido", which can be an adjective or an adverb (meaning "rápidamente"), depending on the sentence, so I think she is wrong, unless this is regional usage considered correct by their local Academia.
