Update GradeSchoolTests.swift

[kalafus]

fix test blocking error: "warning: variable 'school' was never mutated; consider changing to 'let' constant"

[ErikSchierboom]

If I look at the CI run, the warnings are now errors:

error: cannot use mutating member on immutable value: 'school' is a 'let' constant

Should only one tests have been changed perhaps?

[kytrinyx]

As we've not heard back from the original author here, and the tests are failing, I'm going to go ahead and close this.