'Why in the World'

One of the most fascinating experiments in educational TV has turned into fine entertainment as well.

''Why in the World,'' conceived by Walter Cronkite and designed to use current events as a natural and relevant appendage to high school classroom studies, is now being used by hundreds of high schools. More than 22,000 American high schools receive the program's guides, and the show is aired on almost 200 PBS-affiliated stations. Cassettes are available to any teacher who requests them.

What has proven most surprising is the popularity of the show with adults.

It now airs twice a week. (Check local PBS station listings). Recent guest ''guides'' have been Nobel prizewinner Rosalind Yalow, F. Lee Bailey, and Ray Bradbury.

Right now the program is underwritten by General Motors, but executive producer Barbara Barnes-Byden is concerned about the future of the show since the GM funding runs out in May and the state of the economy may spell trouble for corporate-funding plans.