A student at a Brooklyn high school was apprehended today after a knife was found in his bag, police said.

The 17-year-old student was having his bag scanned at Automotive High School on Bedford Avenue opposite McCarren Park in Greenpoint at 9:55 a.m. when the screener spotted the 3.5-inch blade, cops said.

Police ran the teen’s name and found that he had an open warrant for “failure to obey a park sign,” cops said.

The teen was taken into custody. His name was being withheld by authorities because the charges against him were still pending, cops said.