To some extent what sounds natural will depend on just who your audience is. I would probably write that as: While my friends and I were cleaning our classroom, I made a suggestion about cleaning. One of my friends misunderstood and though I was talking about cleaning a restroom. You can omit the last statement about being beside the point as it is clear he misunderstood. If you wanted to leave that in, rather than "beside the point" use "missed the point". "beside the point" can me used when someone misunderstands but, in my experience, that phrase is generally used when someone say something that is just not relevant, even when they have no misunderstanding.