Asterisk is an open source and free telephone replacement system. You still have to purchase telephones and computer equipment, but the software is absolutely free. Asterisk has a reputation for being difficult to setup but solid once it's going. Take it from someone with experience with traditional telephone systems, the only reason they aren't difficult is because the high sales prices includes a complete installation and configuration. You then purchases a maintenance contract to keep it going. If you don't feel comfortable handling a telephone system on your own, there are firms that you can hire to help with the installation and support. Even if you are happy with your current system, Asterisk can be linked to your current PBX to supply voice mail and other advanced services.
You really ought to consider Asterisk when it comes time for your school to replace its telephone system.