This last summer the Countryside Montessori School in Sarasota, Florida, built an octagonal central structure to house their library and serve as a meeting area. To four of the eight walls they fit standard modular classroom units. One wall is used for bathrooms and janitor's closet. Large exterior decking provides convenient outside workspace for the children in this warm weather climate. While this building is still inherently limited by its modular construction, it is much more attractive than many school buildings.
Cost is the most obvious factor.
Their custom designed commons area and decking are something that most schools have not considered in modular construction. Another distinct advantage is construction time. Modular buildings are normally built inside a factory using efficient assembly-line principles, construction is not dependent on good weather. Another plus is the minimal disruption to your building site from start to finish of the new construction. Modulars are set on steel reinforced concrete footings, which are not very difficult to prepare.