"is in" is good English. "is at" is not.
However you should give more details about the watch.
"A watch has been found in Classroom 7A" for example.
The main difference between this answer and your question about the library is that the library is a specific building and can be identified as such. A classroom is just part of a larger building.
Hope this helps
19 października 2019
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We usually use "in" for the word "classroom"
19 października 2019
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