So you've got a two-fold set of requirements: You want to keep sound in as well as making it suitable for those inside drumming.
On the sound treatment for recording/teaching purposes I'd suggest investigating a way to make the drums quieter. I know some other type of drums can be deadened somewhat without ruining the sound too much. Taiki drums - which I've heard many times - are very loud and have a low register for drums (because of their size). That low register is something the sound-proofing would need to consider from the outset.
Typically solid brick/stone/concrete is the only practical way to block low-end sounds. It works better than foam type insulation. In your case you would probably still need a combination of both - to keep the sound in but not make the room unbearable for those drumming. It would need some deadening in the mid and highs and perhaps even a bass trap (google it) in the corner to help keep the sound natural.
Germany has a massive recording industry - one of the biggest in the world thanks to the popularity of electronic music there. I'm certain you could track down someone who could offer advice. I suggest seeking out a sound treatment/insulation products company and ask them. They likely offer some good advice to help potential customers buy the right stuff.
Last edited by Bobby Jr; 02-25-2013 at 02:51 PM.