Fans arriving for the first of a three-game series between the Yankees and the Houston Astros had to pass through metal detectors at Gate 2.
The use of metal detectors at the stadium will be in addition to bag checks and have prompted the Yankees to urge their fans to get to the ballpark earlier.
Major League Baseball has told its 30 teams they must implement security screening for fans by then, either with hand-held metal detection or walk-through magnetometers.
A new online guide lets New York City high school students learn things like whether they need to pass through metal detectors and wear uniforms.
Students began school Tuesday under a heavy blanket of security at North Valley Demarest High School after police got a tip that someone was threatening to bring a gun to school and open fire.