Forever Young (1989)

Five orphans, five friends. They grow up together, still good friends, and travel to Thailand, where a witch doctor has invented a brew which returns one to childhood permanently. Robberts attack and force the five to test the brew. Rescue comes after four of them have shrunk into child sizes, leaving Bo the only adult. The doctor forgets how to reverse the process and the quintet return to Hong Kong. Teacher Wong takes the four children into a primary school.