Blocking SafeSearch preferances?

I work as a network admin for a high school and am trying to find a way to keep students from being able to change the settings on Google Image SafeSearch. I tried adding the SafeSearch url to the blacklist on the firewall but I came to realize that there is not just one url for the SafeSearch page, so needless to say, that idea was faulty. If anyone knows of a way that this can be accomplished, I would love to hear some suggestions. Thanks guys!