I can’t really suggest specifics, because a lot depends on where the school will be built, and also what sort of people are in that area. For example, the high school I went to was the perfect one for the area when it was built, but the city grew so fast that by the time I went there, so many students enrolling made them have to extend portable buildings into the parking areas just to have classrooms for everyone.
I would say that it is very important to have a possibility of expansion, because very few places are experiencing population loss, it’s almost all growth. So having a good amount of land available is a good thing.
One thing I might suggest is an extra annexed building (most likely near the academic areas) of rooms not specifically purposed. Or, because I know that schools are not always funded for such excesses, have that area be used for something that is easy to repurpose, for example extra outdoor eating facilities. Then, when the time comes that the school needs to expand, the ground would not need to be prepared excessively, and the plan would already be made for the building, so it would fit into both the size and the style of the existing buildings.
That is all I can really think of at the moment. The difficult thing is to balance the budget and required things with others that would be good to add, but not necessarily required. I’ll post again if I can think of anything else.