Schools do have diaper changing stations in them but they are most often located in the nurses office.
A special education classroom or preschool classroom sometimes has a room where diaper changes can be completed.
The school has to be licensed to deal with diaper changing so if the school is not licensed to change diapers then they won't change diapers.
In the school I went too the school had a diaper changing table and facilities in the nurses office but for larger kids that needed diapers they would do a standing diaper change.
If the kid needed help changing the diaper or putting the diaper on and cleaning up the nurse would help them.
If the kid could change their own diaper or pull up then they would do so in the bathroom of the nurses office.
For special education kids a teachers aid would usually do the diaper changes and cleaning of the child in a bathroom or in a room made for diaper changes where diapers and supplies would be stocked.