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Never mind, I already saw that you had posted the solution in a different post, but I hadn't seen it before.

I would appreciate your feedback. Do you think that this rebus was too open for alternative solutions and that the official answer isn't any stronger than the alternative answers published in the comments? Thanks!

Well, I do think that my previous answer was on the spot as per what could be seen on the image, an H superimposed on a class(room), therefore: clash, or how you replied, The Clash ;) I'd even like to think that it was an even more fitting response than the intended one (maybe some subconscious slip on your part?), for the painted black area on the wall is a far stretch from a real blackboard, although I do get the idea that the whole thing should be somewhat difficult so that it's fun, but you created this rebus, you know what you intended to hide in the puzzle.

Thanks for the feedback!