Can't a noun in plural form be describedcomplemented with a noun in singular form? ofOf course it can. hereHere are some examples:
These workers make a lot of mistakes when they work since they are new to this job. theyThey 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. theThe 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).