Traditional picnic benches are made of stained wood such as oak or cedar. Cedar is the more expensive choice, but it will hold up better to the elements and bugs. Cedar will also give off a natural aroma. You will need to stain and spray oak for durability. Oak benches are sturdy and durable if cared for properly. You can make wood benches with or without back support.
Concrete benches are made using a concrete form and pre-mixed concrete. This is the least expensive choice and the most durable to all forms of abuse and elements. Concrete benches are usually shaped with a pedestal bottom or as a thick square-shaped bench. These benches are hard to move but you can paint and power-wash them without any damage to the exterior.
Aluminum benches are inexpensive but can rust. Postpone rust by ordering the bench with a rubber finish. Rubber material put onto the aluminum makes the bench more comfortable and durable. The rubber can be a custom color to match the school colors or a standard black. You can place these benches inside or outside and they come with and without back support.
Environmentally conscious schools may want the recycled picnic benches. Depending on the company that manufactures the benches, the materials may consist of tires, aluminum cans and recycled wood. These benches are more expensive than solid wood, concrete or aluminum but they hold up well to elements and they are "green" products.