I've got access to a ballet dance room with wooden floor and mirrors on the wall. The size is about 25 x 30 feet.
 
It's not in use during the weekend and I'm thinking of a quick solution to turn it into a band practice / recording room.
 
Will it be sufficient if I just put some carpets and hang some thick curtains over the mirrors ? The setup must then be easily removed.

A lot would depend on the ceiling. It would be great if it was high and especially if it is suspended tile or irregular. As you probably already know, the worst place for amplified music is in a hard surfaced box. But if the box is big enough and you can stand close to your speakers you can tune out the delay reflections. So the more dead surfaces the better. 
 
If this is a rental possibly the land lord will allow you to permanently install a curtain rod. 
I have a collection of 6x12 and 8x 10 throw rugs we use for both recording and on certain hard stages. You'll notice a lot of big time bands seem to do this. Mine is "the Dudes"  carpet which seem to always tie the room together. 
 
I also have a stash of office dividers ( GoBo's) that are excellent to isolate band member's. Possibly those could be stored at the site. 
 
And if the goal is to be recording, why not invest in a real good headphone system. My Scarlett Mix Control can have a individual mix for up to 6 people. With the right interface or digital mixer you could eliminate a PA altogether. QSC touch mix, Behringer X18 etc. 

Thanks!
 
The ceiling may need some though, it's not very high, just a regular office rental block converted to a dance studio.
 
I'm also studying the Behringer X18 or X32 Rack.

If it's a suspended type ceiling it might be OK, It's the  8' high all drywall( Sheetrock in some languages)  rooms that are impossible without a lot of treatment. 
The X 18 would certainly cover most bands and at almost 1/3 the price of a x32 , if you don't go nuts on drum mikes it might be all you need  Drums can always be sub mixed. I will forgo my tom mikes and  use the overheads when short of channels or submix them all to a stereo pair. 
One thing I'm itching to try is I now have a set of digital drums. So next live drum session I'm hoping to use the Kick from the digital kit. I always end up replacing live kick with Midi anyhow. This free's up an audio channel too as it is recorded via USB/Midi.