James Mumper Jr., 19, was arrested about a month after the Nov. 12, 2022, stabbing death of Mateo Castillo, 17, a senior at Valhalla High School in Rancho San Diego
James Mumper Jr. pleaded not guilty Tuesday to murder in death of El Cajon resident Mateo Castillo early Nov. 12. Another teen boy was stabbed and a third was beaten unconscious in melee among partygoers