Complex sentence: Manny tells his mother about the situation, and Gloria yells at him, but when they enter the school, Gloria notices how sad Manny is and breaks into the girl's locker and slips the locket inside.
Split into simpler sentences: Manny tells his mother about the situation and Gloria yells at him. However, when they enter the school, Gloria notices how sad Manny is, and breaks into the girl's locker and slips the locket inside.