If you have access to the District Office, then the building you would send to by default in the mobile app is the District Office.
However, most students/staff are associated to school buildings instead of the District Office itself. To send to the entire district, tap on the drop down around where it says "District Office" and choose "Entire District" from the Menu.
After you have selected Entire District, you will see the following screen with Entire District at the top. Your alert will now get sent to all of the students and/or staff associated with the buildings that use SwiftK12.