The reason there is a diaper changing station in your high school bathroom is to allow parents to change diapers on their kids during events like Graduation, sporting events and other things.
For example a parent who has a high school student in that school may also have a baby or toddler or young child in diapers still and when visiting the school they would need to change the diaper they could go into the high school restroom or bathroom and change the baby.
I've noticed the baby change stations in high school bathrooms and even in my own kids Elementary school.
I asked the teachers and they said they placed the changing tables in there so parents could change their babies diapers when they were at the school.
Personally I never used them or had a need for the diaper changing station.
I was never at the school long enough for my kids to need a diaper change and would just change them out in the car or van either way or wait until we got home.