High School yearbooks can be a valuable source of information, along with providing a few laughs (when you look at the pictures) for 20th century family members. The information varies widely but the yearbooks often provide you with a ton of story-like information about your ancestor’s life: what their interests were, what sports and clubs they participated in, and often some goofy snapshots of them before they settled into being an adult.
The Ancestor Hunt web site has an article about finding online digitized copies of yearbooks in the western U.S. states at http://bit.ly/2JipbKA. The article contains a number of links to dozens of web sites that contain some yearbooks online. (There is no one web site that has all of the yearbooks.)
Kenneth R Marks is the author of The Ancestor Hunt site and states that he is continually updating the list, so you may want to revisit later.
The article in The Ancestor Hunt is free to read but the links point to some web sites that need a paid subscription and some others that do not. You can find the article at: http://bit.ly/2JipbKA.