Is this sentence gramatically correct?

[Super Sonic]

I wrote that sentence in an exam, and my classmates said that it was gramatically wrong:( Please enlighten me:

"They are a family"

Thanks in advance.

[Carmenn]

''They are a family''....in my oppinion is gramatically correct and it sounds good as well. But i'm a learner like you so don't base on my oppinion.Wait for the teachesr's oppinion.

[mykwyner]

I'm a teacher, and it's a perfectly good, grammatical English sentence (but it needs a period at the end).

[aous02]

You are referring to the individuals who make up that family.

It's perfect English.
What would your smart classmates say .