The name of the previous school attended by the student is already embedded in the
How do I change the name of the school; as my child wish to attend a different school, but a school
that is in the same school system?
If a field remains greyed out within a form, the information present there would have been pre-populated by the district. I recommend reaching out to the Enrollment section of the school district who should be able to assist you in enrolling your child to the school.
I have provided an article here that should help walk you through this process.