It is possible to get your diaper changed by a nurse at school at age 13 or any age if you actually are incontinent and need help changing the diaper.
However even if you're incontinent and need to wear diapers to school then you will be required to change your own diapers at age 13 and above and even less of age.
If you're not actually disabled and can change your diaper on your own then you will change your own diaper at school.
If you're 8 years old or younger and need diapers then the school nurse may change your diaper to ensure you're cleaned properly and ensure the diaper is on correctly.
But at age 13 and above as long as you're capable of changing your own diaper at school then you'll have to change your own diaper.
Or have your parents come change your diaper at school.