Ubuntu Security :: Crontab Disabling Internet From 7pm To 7am?

I administer a desktop computer with ubuntu 8.04 in an university library. Since it works almost all night, to enable students to study, after some time I noticed some misuses of the computer during the evening, when there isn't many students. My goal was to disable users from accessing internet from 7pm to 7am, but also enable it if certain user was logged in (I use that user for torrent, and I seed on that computers from time to time). So I created a script that's being called by root's crontab, and here is the script's code:

Code:
#!/bin/bash
NUM=`who|grep myuser|wc -l`
#echo $NUM
if [ $NUM -le 0 ]; then
/sbin/ifconfig eth0 down
else
/sbin/ifconfig eth0 up
fi

Since I created the script, I actually never seeded anything, so I'm wondering now if that's going to work at all, and (also) is there a better solution for this.