Middle school students and older may be actively looking for naughty content on the Internet, but most younger students simply stumble on it. While filters can help, they are not perfect. In Google, for example, one can still see the image icons for sites that one may not be able to visit. Even with the Google safety filter turned on, a great number of questionable images can appear.
The Zoo.com search engine goes a few steps beyond Google's safe search to make sure the content is acceptable for children. And, there are no images. The chance of accidentally hitting a bad site is greatly reduced. Combine this with a halfway decent filter and your school should be reasonably protected.
One more thing: you're not giving up Google to use zoo.com. They search Google and Yahoo! to obtain the search results. They just clean up the results list first.