I don't know what "intermediate class" exactly is, but your sentence should be something like 「僕は中級の日本語というレベルが達して、アルバイトするより日本語を勉強する頻度が高い」
@YasashiiEirian there are most likely many other ways to say it but I think my suggestion would be more likely. Though I might be wrong in my interpretation of your sentence.
An intermediate class is anything beyond 初級 I think. 中級 is included in a lot of textbook names, but I'm not sure the word is instantly recognisable in conversation.