I've been using OpenDNS for quite some time and recently started using their filtering system.
First, DNS is the system by which URLs, such as abc.com, become the IP addresses (18.104.22.168) that the Internet actually uses to get from one place to another. While DNS is an open source application, OpenDNS is not so much open because of DNS being open but rather that it is not tied to any given ISP. Most users on the Internet use either their company's DNS server or one supplied by their Internet Service Provider. ISPs didn't always spend a lot of money making their DNS servers fast and reliable. In come OpenDNS with their DNS servers. They have great up-time and the look-up speed getting the URL turned into something meaningful has been great.
OpenDNS makes it money mostly by displaying advertisements on the error pages it presents when the user mistypes a URL. You're going to get an error page -- and your ISP is probably going to have ads too -- so that's not a big deal. They also route Google searches via their server. I assume this is the type of revenue system that FireFox uses to get money from Google for any Google search performed from the FireFox search window. FireFox makes millions a year off that search box. And, I'm glad that they do make money. It's not loss to me and I am able to support a service that benefits me.
OpenDNS is a nice addition to a school's ability to block bad sites. The way this works is being creating an account with OpenDNS and telling them that you want searches from your site to be filtered. You can select differing levels of protection from nothing to very tight. Any student trying to reach a bad site either intentionally or unintentionally will get an OpenDNS block page.
The system is not a perfect solution. If you want your staff to have open access from the same network as the students, the same restrictions would apply. I don't think this is a huge concern given that even the most restrictive filter setting does not appear to block anything truly useful.
Another problem with using only OpenDNS to block bad sites is that it obviously cannot help you if your student knows the IP address of a site they want to reach. OpenDNS would not be involved. It would not be a lot of fun for the student to get around the system in this way, but it is technically possible.
And, some schools may have issues with ads being displayed. I don't think there is a work-around for this issue. Given all the ads that are in school materials and on vending machines, I would think these ads are no worse, but don't assume you're okay until you have asked.
I would recommend using OpenDNS as one element -- a free one -- as part of an overall Internet safety system. You should also tell the students' parents to use it at home.