The opposite of "as soon as" in this context is: "soon after". This is because "as soon as" expresses how the chanting started immediately the headmaster appeared. However, "soon after" expresses how this chanting the not start as soon as he appeared but after he had already appeared. "Much later" is a close second but, it cannot appropriately fill the slot that "as soon as" fills - it will create a grammatically incorrect sentence: (Much later the headmaster appeared, the chanting resumed).