Are they primary or secondary school children who would just scrap the surface of Moodle or are they technical freaks who need everything and more from the LMS?
21,000 is a big dimension. Like many other fields of engineering these dimensions do not scale linearly. Making 1m long "brige" is one thing, even a 10m culvert is something completely different. A 100m brigde needs a technological breakthrough, and so on.
For a single class, one can run Moodle on any PIII machine and be happy. Scaling upto 21,000 have to cross quite a few barriers, but even now, if your calculations go on Dollar/Student basis, your budget too must be of a different size
And then how are these students distributed, geographically and in usage-hours? What sort of support they need? How about the user management, keeping 21,000 accounts itself could be a bit a full time job.
How about the content? Do you have them already? Do you expect the present teachers to add their content to the LMS? What support they need?
As suggested by other posters, this is a job for professionals. Of course you are welcome to discuss your szenario to be able to make a judgement yourself.