The Brain (movie, 1988)

Dr. Blake runs a TV show called "Independent Thinkers", which is sort of a Scientology-like self-help/religion program. But he's not making his audience think any more independently - with the help of an alien organism he calls The Brain, he's using brainwashing and mind control. The only thing that stands between them and world domination is a brilliant but troubled high school student with a penchant for pranks...