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.
These are called “set phrases.” They popup every now and then in almost every language where one way of saying something just becomes more common than another perfectly valid way of saying it. There are likely a number of different factors that causes us to do this. One is that it can be easier to understand a phrase if you say it consistently the same way as people around you do. Another is that a can servative as a Shibboleth, something that helps you to recognize outsiders.
“The correct preposition to use depends on the context.
We use “in class” when we are referring to being physically inside a classroom and participating in the activities that are taking place. For example, “I am in class right now.”
We use “at class” when we are referring to attending a class or being present at a specific class session. For example, “I will be at class tomorrow morning.”
So, both “in class” and “at class” are correct, but they are used in different contexts.”
-Stolen from Quora, thought it was a good answer.
That said, prepositions are fickle. “At work” is usually used because it implies there’s a workplace that you are at. Not all work takes place “in” anything, so it’s not so commonplace.
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