[Chris_Ely]

PVC pipe is NOT RATED FOR AIR PRESSURE!
PVC pipe is rated for hydrostatic pressure only. Using PVC for containing pressurized gas can lead to serious injury or death. When PVC pipe fails due to pressurized gas, it does so catastrophically.
There is a big difference in air and water pressure. Water is incompressible. If a pipe bursts holding water, the water will just leak out. Air on the other hand will expand to reach the same pressure as the atmosphere, releasing an enormous amount of energy, which will propel shrapnel at high speeds in all directions.
Don't just take my word for it, see this engineering document from Charlotte Pipe, this recommendation from the Plastic Pipe Institute, and this warning from OSHA.

Originally Posted by BBray_T1296

People distrust hand-built pvc as a pressure vessel, but we have successfully fired hundreds of T-shirts at 155 PSI (Firing nearly clear over our stadium [which is huge]), without incident. It was PVC cemented on the day we built it and has had zero maintenance in the 7 years we've had it.

This does not mean it won't fail the next time you use it. Fatigue cracks have most likely started, which severely reduces the strength of the material.
What is more important, quick and cheap air cannons, or your and your student's safety?