The two sentences are both valid sentences, but would be used in entirely different contexts.
In the は example, you are highlighting the topic of your conversation. Suppose you are in the cafeteria with your friend, and you want to say "Japanese class is interesting, isn't it." You would use particle は thus:
You are introducing a new topic of conversation, and so the は particle hilights what it is you are talking about. A literal translation of this is something like "Speaking about Japanese Class, interesting, right?" I find using "Speaking about" as a translation for は helps to remember it.
が is used to highlight the specific subject of a sentence, when you want to clarify what it is you are talking about inside of a specific context. For example, your friend is asking you about the classes you are taking at college:
"Speaking about your college classes, which is interesting?" In this case, replying using the sentence above with は would sound unnatural, like you are changing the subject (which you are because は introduces a new topic!). Instead, you would reply naturally:
This は・が distinction extends to other things, for example the simple "I like" sentence. Say your friends are talking about what fruits they like, and you want to chime in by telling them you like apples:
"Speaking about me, apples are liked." In this case, the topic of conversation is you, and what you like. The specific subject of your sentence is apples - that you like them. You would drop the
私は if someone explicitly asked you which fruit you like - you don't need to change the topic because they are already talking about you.