There are multiple ways you can test the issue Jeffrey.
- Load the Project aia on your personal laptop and try the Project from you home router. If the Projects runs ok, the issue is probably Internet congestion and/or an over-stressed school network (or you could just be testing while the traffic on App Inventor's server is less)
- share the aia with this community and someone will run the app and tell you what happens.
Is it possible that your IT department made recent changes on its 'white list' recently - i.e., is something not allowing the school network to work with mp3 or png/jpg files?