Do you need a mobile app, or just a mobile-optimized website?
Some ways an app can add value:
Push Notifications (users may be overwhelmed with email - get through the noise and avoid the spam folder)
Offline access to information (connect with parents and students on the move with wi-fi tablets, slow connections/out-of-range, etc.)
Deep device integration (social media sharing, calendar, reminders, etc.)
Redesigning an existing website for mobile can be a big project, so signing up for a customized school app from a specialized service provider like Appazur may be less expensive.