> there will be around 25,000 users and perhaps 5000-8000 simultenous users access
You expect 30% of your users to be active at peak times? Isn't that a bit high? What is the explanation? Usually one idea of online teaching is that people work whenever it is convenient for them. As a restult the load gets distributed around the clock.
In any case, 5000-8000 concurrent users is massive! Not only you need big-iron, but also employ clusters. There were couple of dicussions in this forum with various architectures. Do a search (search field top right).
Do you people already master a clustering technology?