Why does Brother Leon tell Archie that he (Leon) will soon become Acting Headmaster in "Chocolate War"?
Brother Leon will soon become Acting Headmaster because "the Head is ill, perhaps seriously so...he's schduled to enter the hospital...the outlook isn't good". Brother Leon tells Archie of his promotion for a couple of reasons. The first is to drive home the point that he will be in charge. Brother Leon is soliciting the help of the Vigils, the underground organization headed by Archie in supporting the upcoming candy sale. The Vigils "(serve) a purpose...(they) keep things under control", but being the type of organization that they are, the school cannot condone their existence. In actually, the school "allow(s) it to function by...pretending it (isn't) there". By asking for the Vigils' cooperation, Brother Leon is acknowledging the group's power and giving them unprecedented leeway. He is saying that, since he will be in control, they will be able to operate with impugnity, without fear of ordinary disciplinary reprisal.
Brother Leon is a weak man, but he is also ambitious. He is the Assistant Headmaster of the school but actually serve(s) as a flunky for the Head". Brother Leon is inordinately proud of his promotion to the position of Acting Headmaster, and relishes the power it gives him. A second reason behind his decision to reveal his ascendancy to Archie is arguably to boost his own ego by impressing a student whom he knows to be influential (Chapter 4).