What are you counting? A set -> is on the table. A group -> is in the classroom. "A..." means "one of", so the only possible answer is a singular verb. "Of ____" does not affect the verb. That's the basic rule you should be following.

Rajalala has a fair point: if you consider the subject as "they" (eg. "my family" can be treated as either a single group or plural people), then you might get away with swapping is/are... you "might". Still, you shouldn't be careless with grammar rules, because sooner or later someone will pick you up on it! :)

Thanks. I've learnt a lot.

These rules can vary between US English and UK English. In my understanding, "the team were brilliant" would be correct in the UK as the logic of the grammar is different. "The" is singular but a "team" is comprised of many people, thus the verb would be in the plural.