I am brave enough to take the chance:
Microphone:
Røde NT1-A
Studio condenser, neutral clean sound, low noise.
USD 230 at Sweetwater.
Plus a mic stand and a pop filter.
Audio interface:
Presonus Firebox
Firewire, good sound, 2 Mics and 2 line in, 6 line out, phones out.
Currently USD 200 sales price at Sweetwater, retail price USD 500.
Speakers:
Yamaha HS80M
Good value for money.
USD 800 for a pair at Sweetwater, cheaper in Europe.
Plus solid speaker stands.
Headphones:
AKG K-171 or equivalent (Beyerdynamic for example)
USD 200 at Sweetwater.
You need closed phones for voice recording!
Be aware that you have normally only one phone output on an audio interface. If you need two or more phones you can take a small phone amplifier and may need an "Y-Cable" from two interface outputs to the phone amp input.
Cables:
One long mic cable, XLR/XLR, 6 m minimum for a class room. Two symmetrical cables from the interface to the speakers. In case of the Yamaha speakers you need symmetrical cables, either TRS/TRS or TRS/XLR male. Probably extension cable(s) for the headphones.
So - where did we go?
I think including stands, cables and shipping we settled below USD 2000. The speakers eat up a lot, you may want cheaper models. Just remember that small speakers don't have much bass and in the classroom you need speakers that can deliver something in the mid-range field, at least not dedicated nearfield monitors.
I apologize if this budget is far too high for your application. This is for a home studio setup, allowing to record and mix in good quality. If you are rather after a "demonstrational" setup you have to take another approach and I don't know much about gear for that (rather nothing at all).