A 15-year-old boy was slashed in the face, possibly with a box cutter, inside his Bronx charter school Thursday — the third burst of Big Apple student violence in as many days, according to cops and police sources. The unidentified student was rushed to Lincoln Hospital in stable condition following the attack at the AECI Charter School in Melrose, cops said. Another 15-year-old boy was taken into custody, but no further details are immediately available, police said. The two students were “fooling around” when one of them pulled out what’s believed to be a box cutter and slashed the other’s...