Columnar European Hornbeam would be a good tree to use. Consider keeping them out closer to the walk in small circle beds in the lawn. Break up the monotany by keeping the grass going all te way to the building in some places and adding beds in others.
Keep in mind that it is a school, so they are not going to want places for people to hide whether it is a college campus or elementary school. That means hiding to break in through a window, or hiding to ambush, or hiding from someone.
It is a big building, so mass plantings in bigger groupings is very effective while detailed plantings of indivisual plants will look very spotty.
Think "big picture" and abstract rather than about details and it will be fine. The building is so diminant that detailed plantings will bo un-noticed except at entrances and seating/meeting areas. Design to affect the building first and the people using it second in this type of situation.