REVERE – An alert Revere police officer caught three graffiti vandals red-, er, black-handed Monday night, police said.Officer John Chann happened to be driving behind the Garfield School at about 8 p.m. when he noticed three people spray painting the building. He said one of the suspects, 19-year-old Alexander Shaffer, had black paint on his hands when he was placed under arrest.Shaffer, of Loredo, Texas, Jennifer Nguyen, 20, of 53 Elliot Road, Revere and Andrea DeLeon, 20, of 52 Buswell St., Boston, were all arrested and charged with trespassing and graffiti.Police noted in a report that several “No trespassing” signs are posted in the school yard.Chann reported that the youths were vandalizing the part of the building that faces the railroad tracks. The suspects initially tried to walk away when they realized a police officer was present, police said.The officer noted that the suspects appeared to be spray painting the shadow of a tree when he arrived.