Big Brother is joining the battle of the bulge. A group of Long Island students will soon be wearing controversial electronic monitors that allow school officials to track their physical activity around the clock. The athletics chair for the Bay Shore schools ordered 10 Polar Active monitors, at $90 a pop, for use starting this spring. The wristwatchlike devices count heartbeats, detect motion and even track students’ sleeping habits in a bid to combat obesity. The information is displayed on a color-coded screen and gets transmitted to a password-protected Web site that students and educators can access