Henry Siebor, 42, a Polish immigrant and construction worker living in Greenpoint, fell four stories to his death yesterday when a scaffold on which he was working in Park Slope collapsed at one end. Siebor was wearing an untied safety harness; the harnesses of his two colleagues on the scaffold were tied, and they survived. City officials are investigating the rigging of the scaffold .. A 59-year-old school bus driver suffered a fatal heart attack and died while at the wheel in Queens yesterday; a counselor and nine kids from the Magic Carpet Day Camp were aboard at the time. One of the kids jumped into the driver’s seat and pulled the emergency brake, which is thought to have limited the damage. The kids are alright but a bus matron suffered minor injuries… A taxi crashed near the 72nd Street subway station entrance at Broadway just before rush hour yesterday. One pedestrian’s foot was hurt by flying debris, and the driver and two passengers suffered minor injuries. Witnesses say the driver, who swerved to avoid another car, was speeding… John Cedeno, 43, got out of his SUV to let his 13-year-old son try parking it on East Tremont Avenue in the Bronx last night. The kid threw it into reverse and pinned his father to a tree, killing him.