In what way does Jack discredit Piggy and Ralph in Lord of the Flies?

Jack basically discredits Ralph and Piggy every chance he gets. Jack knows that Piggy is an outsider that the boys consider weak and a victim. Jack takes every opportunity to connect Ralph with Piggy to undermine Ralph's authority. Jack does this at meetings, exploring the mountain top and at their pig roasts. Any friendship that Ralph shows to Piggy is put into a bad light. One of Golding's ironies is that the only time the boys, except for Simon, seem connected is when they are humiliating Piggy. Even Ralph finds this hard to resist.

Just to add, A good example of Jack's strategy is after Ralph admonishes him for letting the signal fire out while a ship passes. Clearly the boys were realizing the enormity of the situation. One by one they begin to cry. Jack deflects this negative attention by punching Piggy. Piggy's reaction brings most of the boys into a more jovial spirit. Jack, very half hardly, apologizes for letting the fire out. The boys think that this is all very proper of Jack and the tables are turned on Ralph. Ralph ends the scene by looking at Jack and saying, "That was a dirty trick."