If you are looking for extremely short plays, 10-minute-plays.com is a good place to check out. They offer an eclectic collection of short plays with running times ranging from one minute to 10 minutes. The plays are for sale. Each play lists cast and set requirements and notes the running time. You can read the plays in their entirety before you purchase them, so you will know what you are getting. These plays are a combination of originals by current playwrights and adaptations of classics. Some of the titles include "The Serpents Tail," "Phone Arts" and "Blue Christmas." This website also includes a description of how short plays are structured and discusses writing your own short plays.
If you are looking for suitable one-act plays for high school production and are tired of doing some of the same plays that are always performed, check out Theaterhistory.com for suggestions for one-act plays you can produce for your high school. The website also lists some of the most often performed plays in high schools and competitions, in case you want to stick with something tried and true. Some of these titles include "The Actor's Nightmare," "The Bald Soprano" and "Am I Blue."
The Comedy and Drama Play Index is an extensive library of three- and four-act plays available for performance. This website offers modern and classic comedy and tragedy, listed by author. Some of the works include plays by Winston Churchill, O. Henry, Edgar Allan Poe and George Bernard Shaw.