Public Schools of North Carolina State Board of Education: Making Current Trends in School Design Feasible, November 2000: Smart Growth, Joint Use, Sustainability, Other Design Issues. See in particular the first paragraph on page 37, quoted below:

Shared Inter-Scholastic Outdoor Athletic Facilities

Similar to, but perhaps more extensive than the approach for middle schools, several high schools, or even an entire district or conference, could share football and baseball stadiums, as well as a competition gymnasium with a large seating capacity. Competition athletic facilities are one of the most expensive and land-hungry facilities associated with high schools. From an economic standpoint, it is difficult to justify a 4,000-seat stadium, complete with concession stands, very large restroom facilities, lighting and other amenities, that is only used for four or five games a year. Likewise, providing seating for 2,000 or more at an indoor gymnasium significantly increases the size and cost of providing a physical education program. Multi-purpose practice fields would still be needed at or adjacent to each school, but these fields can be significantly less elaborate and do not necessarily even need to be full-sized.