This is one of the grammatical rules where it is much easier to “feel” what is right based on how we perceive the world rather then try to analyze it. When you think of work or school you think of a place or location which means you should use “at”. But when you think of class or office you think of the room so you should use “in”. Basically you are inside the room but you are on top of the location. Again this is hard to analyze logically.
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