Re: [HELP] Only fourth core used on quadcore?!
While the Teeworlds server isn't itself a multithreaded application and won't run on multiple cores/processors, multiple instances should be scheduled to run and balance the load over the available computing resources.
My guess is that the problem is with your installation and not with Teeworlds at all. You would have more luck asking to any Debian support related corner.
Also, the problem has nothing to do with the usage of screen even if it's not required. You can very well run the server with nohup and redirect its output to your logfile or /dev/null and it will not shutdown when you quit the parent shell.
PS: You shouldn't run the teeworlds server at each cron call; check if the processes are running using ps or pidof.