Question: I'm building classrooms in the large basement of our home; we're in Spain and have an English school we're expanding. 

Room to room sound insulation and resonance are the major concerns I have to consider, as well as costs. The actual ceiling is over three metres high and I plan on using acoustic ceiling tiles to help with echo, which currently is a major issue. 

Does it make sense to build dry wall, say, two and a half metres tall with suspended ceiling lowered inside and insulated above with glass wool or similar? That would mean there would be a large open space above the classrooms which would be sound insulated from below. This would save (insulated) dry wall costs and glass wool is pretty cheap.

Answer: 
If sound proofing is your primary concern then you should build the walls all the way up to the roof, past the suspended ceiling. You should insulate the walls with sound-dampening insulation. If you are willing to spend more money and time, you should look into isolating channels that the drywall attaches to. This prevents sound from being carried through the walls and studs (normally sound hits drywall and vibrations are carried through the studs to the other side. The isolating channels prevent this).

I do not think a suspended ceiling will be able to hold the weight of the insulation. As well, usually a big benefit of a suspended ceiling is easy access to the plenum space above for running wires, accessing hvac/plumbing/fire equiptment, etc. If you have insulation on top, everytime you need to take a panel out, you'd also have to take out a lot of insulation. It would also be hard to get it back in properly after removing only a single tile. The tiles themselves should help eliminate the echo.
In general this is valid. I've done industrial networking, and this design is pretty prevalent. What I'm not sure about it where you are putting the "glass wool" insulation? Are you insulating the unfinished walls above the ceiling? Or were you putting the insulation flat on top of the acoustic tile?

It the latter, then it is imperative that it be kept dry. Those ceilings can't support much weight, and heavy wet insulation could cause it to come crashing down. Keep in mind that acoustic tile is already sound insulation.