A masked gunman shot and killed the driver of a bus and two young passengers near the town of Decani in western Kosovo on Friday evening.
A third student was injured, media in Kosovo reported.
Police have launched a large-scale operation to find the shooter, according to Kosovo National Police Director Samedin Mehmeti.
Those passengers killed were a schoolgirl and a schoolboy aged 17 and 16 respectively.
Witnesses said there were fewer than 10 people on the bus, which was heading from the town of Djakovo to Decani. The attack occurred near the village of Glodjane.
Police said they did not think terrorism was a motive.
President Vjosa Osmani and Prime Minister Albin Kurti expressed their shock and called on the police to catch the gunman and bring him to justice.
Interior Minister Xhelal Svecla cancelled a visit to neighbouring Albania due to the incident.
