Find a location that receives plenty of sunlight throughout the year. The south or southeast side of a building is usually best. Morning sun is most important because it allows the plants to start food production early in the day.
Determine the type of greenhouse you want to build. A lean-to greenhouse, which can be built along the side of a school building, is inexpensive and can use heat, water and electricity from the building. A free-standing Quonset, or rounded, structure permits more location options but requires independent heat, water and electricity systems.
Decide on materials. Frames can be made of wood, steel, aluminum or PVC plastic. Coverings can be glass, fiberglass or film plastic. A PVC frame with film plastic cover is the least expensive and easiest to build. Your foundation and floor should allow for effective drainage. A gravel floor is recommended.
Build your foundation. If you are using glass or fiberglass, pour a concrete foundation. Greenhouses with plastic covers can use a wood foundation anchored by posts set into the ground.
Build your frame. You can buy prefabricated greenhouse frames, which may be easiest if you are building with steel or aluminum. An inexpensive option is to build a Quonset style frame with PVC pipe. The PVC is anchored by 30-inch lengths of rebar, which are driven halfway into the ground along the foundation. Slide the PVC pipe onto the rebar and secure it to your foundation with metal bands. The PVC can then be bent to form a frame with a rounded top. A lean-to greenhouse will look like half of a Quonset built against the side of a building.
Cover the greenhouse. Glass is attractive but is the most expensive option and requires the most rigid construction. For a school project, a plastic film covering is much less expensive and easy to maintain.
Install heat and water systems. Your heating system will depend on your climate and the types of plants you want to grow. The watering system can be a hose run from the nearest building or a more sophisticated system using sprinklers, drip units and timers.