What's the difference in sense?

The way I see it, if somebody asks: "How many students are in the classrom?" it's because the total number of students is known by the speaker, and he/ she wants to know how many of those students are in the classroom.

Whereas if somebody asks "How many students are there in the classrom?" is because the students are not known by the speaker.

Is it right? I'd like to hear from native speakers.

Thanks a lot.
I think there may be a bias towards the differentiation you suggest, but for me it's nowhere near clear-cut. Your first sense is more apparent in "How many of the students are in the classroom?"
Yes, you are right, I see it. If I wanted to convey the first meaning, I'd use "how many of the students".

But then, how could we explain the difference between both sentences if there's any at all?
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There is no strong or conclusive difference in meaning that I can identify. At best, there are slight or debatable nuances, such as the one you identified. To me, the version with "there" has a vague sense of describing a more "static" or "definite" state of affairs. I would also guess that in everyday spontaneous speech, the version with "there" is more common.
This is an interesting distinction.

It does seem that the version without "there" is drawing a contrast.

How many students are in the classroom compared to students in the hallway.
How many students are in the classroom compared to the number of teachers in the classroom.
How many students are in the classroom compared to the number of boxed lunches we have to give out.
Barb, I think you meant to type "drawing a contrast."
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I most certainly did and I will correct that now. Thank you for pointing it out - I shudder to think how many people read that and though "What the beejeezus does it have to do with contracts?"
Thank you all for your responses.