Is "Everyone are happy today" correct?

Why

Nandini's reasoning from MEANING makes complete sense — "everyone" really does refer to a whole crowd of people, not one single person. English grammar, though, treats "everyone", "everybody", "everything", "each", "either", and "neither" as grammatically singular by form, regardless of how many people they refer to in meaning — the word is built as if it means "every single one", considered one at a time, which is exactly why it takes a singular verb like "is". This is another case, like "news" earlier, where a word's grammatical behaviour doesn't match what feels intuitive from its real-world meaning. Test: notice "everyone" pairs with a singular pronoun in careful writing too — "Everyone should bring HIS OR HER book", not "their books" in strict traditional grammar — the singular treatment runs consistently through the whole word's behaviour, not just the verb. Her friend is right: "everyone" is treated as singular, so it's "Everyone is happy", not "Everyone are happy", however many people it actually refers to.