"Because the class is difficult, I study at home."
I was also looking for an answer! Here's an excerpt from my JLPTN5 book: "Ga follows the subject of a sentence with an adjective" This can probably help more too: http://www.japanese-language.aiyori.org/article1.html
I was strugling with this but then I remembered ga is for something that has acition on it, so school が dificult. For example, me studying at school - school has no action, it is a place where something is happening so, school に me study. And then school is participating in a contest. School is subject with no action on it so, school は contest participate. Someone correct me if this is wrong.
Doesn't seem to accept クラス. 授業 for me carries more of the nuance of 'lesson', but even if not, クラス is correct Japanese and should also be accepted: https://jisho.org/search/%E3%82%AF%E3%83%A9%E3%82%B9 vs https://jisho.org/search/%E6%8E%88%E6%A5%AD