A 17-year-old student charged with first-degree murder in the slaying of a Christian school principal told investigators he had planned to stab the woman on the third day of classes, when he knew he’d be alone with her in a classroom, police said Thursday, NPR reports. Eduardo Marmolejo was held without bond and ordered to undergo a mental evaluation by a Juvenile Court judge during a hearing Thursday. State law allows police to release the names of juveniles charged with first-degree murder…