Schools were not built to provide access for telephones. Installing a telephone system can be an expensive proposition. Given that most teachers have cell phones, why not use them? Most people have calling plans that make local calls free. Using cell phones also means that your staff can be reached regardless of their location -- even outside the building.
The main problem with cell phones as an alternative telephone system is reception. A system that requires a teacher to find a window to stand next to is not going to work. This is where having a cell phone signal repeater comes in handy. It takes a signal from the outside of the building and transmits it to the inside of the building.
The repeater has the ability to support multiple antennae cables. This allows you to wire up different floors or sections of a building using just one repeater. One downside is that if you want to support multiple cell phone vendors, you may require more than one repeater. Each repeater is configured for a specific frequency range. So, for example, T-mobile and Verizon may use different frequencies. If so, you would need a unit for each. Fortunately, they both can use the same internal cables to transmit their signals.
Oh, and one more thing: this system will make it easier for your students to use their cell phones too. The repeater does not have the ability to recognize specific cell phones.