Here's why:
* Subject-verb agreement: The subject of the sentence is "a large number of boys," which is plural. The verb "was" is singular.
* Collective noun: "A large number" is a collective noun, which can be treated as singular or plural depending on the context. In this case, since we are talking about individual boys being caught, the plural form of the verb is needed.
Here's the corrected sentence:
"A large number of boys were caught copying."