We did pricing analysis with VOIP hosted such as Sovernet - and even with Erate discounting, the cost after 10 years was substantively higher. 10 years is a good comparison point as it is the lifespan of an installed pbx (but most schools wait until 15 years). An installed solution is paid off after the financing period, then just maintenance. Plus you don't need internet redundancy, which you would want if you had a hosted pbx.(I would consider this essential for emergency purposes - copper does not provide the E911 compliance in emergency outage of Internet). Reliability is a key factor with an on-premise solution.
Plus installed is more flexible . You could do digital or analog to classrooms and still be E-911 compliant with an installed solution. It does not have to be ALL IP. but with Hosted - you need wired Cat5E or 6 to all locations - big wiring bucks and switching is no small component.