grammatical number - Is it correct to say: These are not homework?

A teacher sent home a list of assignments with a cover letter explaining, "These are not homework."

"This is not homework," or "These pages are not homework," sound equally normal to me, but "These are not homework," just sounds weird. Is it correct grammar?

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I don't agree that the question: 'Agreement in “[Singular Noun] Is/Are [Plural Noun]”?' describes this specific usage. In "These are not homework," the word "These" is not a singular noun. It's a plural pronoun. I suspect that some of the problem is that the missing noun is implied to the reader only by the physical presence of other documents, and not contextually from the surrounding content of the cover letter itself.

Answer

Can't a noun in plural form be complemented with a noun in singular form? Of course it can. Here are some examples:

These workers make a lot of mistakes when they work since they are
new to this job. They are not the main reason we are losing money – the state of the market is.

and:

These people are my family.

and also:

We are a team!

If the above sentences sound grammatically correct to you, there's no reason why your teacher's sentence would be any different.

The quote you provided is perfectly fine and makes the same sense:

These (things you need to do) are not (the) homework (you are
obligated to do).