The incident occurred on Wednesday afternoon in Dalarna, central Sweden.
The teen is reported to have fallen from the truck, hit his head on the ground, and then been run over by the truck. The extent of his injuries remain unclear, but local police said the truck rolled over his leg.
A reporter for the Dalarna Tidning newspaper said nearby students were in tears. The road was cordoned off by police.
Students who have finished their senior high school year in Sweden traditionally pile into the back of trucks and ride around town, dancing to blaring music. Often, a great deal of alcohol is involved.
Last week, a similar parade was cancelled
in central Sweden after two students fell off one of the trucks. Police on the scene said the students' drunken behaviour was "worse than ever before".