The nuns who teach at the Convent of the Sacred Heart girls school on East 91st Street are said to be aghast at the exploits of one alumna, Stefani Joanne Angelina Germanotta, a k a Lady Gaga. The pop tart is known for her scanty costumes and such lyrics as “I wanna take a ride on your disco stick.” Said one source, “When someone showed the nuns a video of her bloody performance at Sunday night’s VMA Awards, the good sisters were not amused.” A school spokesman didn’t return calls.