Unless the child has some form of disability or bladder problems that require the child to wear and need to wear diapers then the child must be potty trained and out of diapers before being enrolled in Preschool or Kindergarten.
If the child has special needs or has a bladder problem that requires the child to wear diapers then the school will make special arrangements for the diaper changing for the child or may require the parent to come into the school to change diapers.
I've read online that some children in the UK are going to school in diapers and they're hiring people to change diapers in schools even for kids that are not special needs.
But in the USA kids need to be potty trained before going to preschool, kindergarten or any other grades in schools.
But like I said if your kid has a bladder condition or has a special need and cannot use the toilet and has to wear diapers then yes they can start school wearing diapers.