I saw this on a local forum.

All of his classmates is gay! (commonly used in UK)
All of his classmates are gay! (commonly used in US)

Is that true ?

No, say 'are' with a plural.

But you could say 'all of the sugar is in the kitchen'.

Thank you Clive.

Do you mean that it is not commonly used in the UK?

Is it possible to have a native speaker from the UK to confirm whether "All of his classmates is gay!" is commonly used in the UK or not?

Thank you.
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Only are.

Go to Google:

and search yourself over the BBC site by using for search the following:

site:bbc.co.uk "his classmates are"
then with
site:bbc.co.uk "his classmates is"

Thank you Marius.

I have searched that on Google. There is no item in "his classmates is". There are six items in "his classmates are" but none of them comes with a "all" . Therefore I think it is not conclusive in this case. Anyway, I will take your words that it is wrong to use "is".

Thank you again for your help.
It is conclusive in this case, Hongkie.
"All of his classmates is gay" is just as grammatically wrong in British English as is is in American English.
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Thank you Yankee.
AS a Brit I will confirm that we say 'are' here.