A group of pupils are lining up at the canteen.
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Vincent Teo"group" is grammatically singular but semantically plural. In British English we can use either "is" or "are" in such cases, depending on which "seems best". In your sentence I prefer
What do you think if I say, "A group of pupils IS lining up at the canteen.
We are referring to a group, which is a singular noun. Am I right?