As soon as sixth grade is out, Alex Santacroce drops his black Air Jordan backpack in the house, grabs a banana (good for hand cramps) from the kitchen, and mounts his custom-built SE Savage Flyer bike. He can appear almost solemn as he coasts down the driveway, front wheel aloft, gently shifting his body to keep the handlebars straight. Riding is absolutely mind-clearing for him. He says sometimes he’ll try a trick 1,000 times before it’s perfect. To some residents and the Haverford, Pennsylvania, police, Santacroce, 11, is the neighborhood’s primary menace. It is not only that he and his sib…