What's written behind the makeshift club sign?
The original sign is 生徒会室 seitokai-shitsu "student council room". 

I haven't read the manga, so I can only presume that their club has co-opted what was formerly the student council office as their headquarters for some reason. 

(I guess this is something of a tip-off to the episode 1 twist - it's weird that another club would just up and occupy the student council office, after all.)