Panicked parents heard gunfire and rushed to the scene at around 3pm on Tuesday afternoon.
The victim was sitting in a Porsche car on a busy road when the attack happened near St Mary's Grammar School.
In a statement, the school said the man was collecting his son, a pupil at the school.
"The thoughts and prayers of the principal and all the staff are with our pupil and his family at this very sad time," it said.
A man aged in his 40s wearing a high-visibility vest is being sought by detectives after the incident on the Glen Road.
The car was covered in a tarpaulin as forensics officers combed the area for clues. The Glen Road was dominated by cordons, police land rovers and flashing lights.
West Belfast MP Paul Maskey said: "It is deeply concerning that this killing was carried out in full view of hundreds of kids who were leaving school at the time."
SDLP Belfast City Councillor Tim Attwood added: "It is absolutely sickening that a gun was fired in the vicinity of both a primary and secondary school in west Belfast today.
"A number of pupils witnessed this fatality and are understandably very disturbed and frightened after this reckless act occurred nearby the school gates.
"There is absolutely no place for this in 2018; this madness belongs in the past and we cannot allow it to once again become our normality."
Detective Chief Inspector Peter Montgomery, of the Police Service of Northern Ireland (PSNI), said: "Police are currently at the scene of a shooting during which a man died this afternoon.
"I would like to appeal to anyone who saw a man in his 40s wearing a high-vis vest with the words Security on the back heading in the direction of Glen Road, as well as anyone who witnessed the shooting or was in the Glen Road area from 3pm today, to come forward and speak to police."
