classes school []; is illegal. You must specify a size (that is a compile-time constant) for the array. You may want to disable the compiler extension that allows it.
In readSchool (school[5]);school[5] is an element of the array. Since you didn't specify a size for the array and your array has no elements, 5 is an invalid index into the array.
The proper way to call readSchool (after supplying a size on line 23) is: readSchool(school).
Change line 29 readSchool (school[5]); to readSchool (school); and change line 23 classes school []; to classes school [5]; school should be a local variable in main, but thats up to you. Note this code won't compile until you define your readClasses function
Just to clarify things - your error message was telling you that you were trying to pass an object of type classes (school[5]) into a function expecting an argument of type classes* - that is a pointer to classes. The two are different types.