What did Susan's (Stargirl's) father do with the silver plate?
Stargirl's father gives the silver plate to Leo.
Stargirl had won the silver plate by placing first at the Arizona State Oratorical Contest. The students at Mica High had been shown a film during assembly period showing a past winner of the contest returning to a hero's welcome at school. Stargirl has a vision one day of being in that position; she pictures herself "returning in triumph from the Arizona State Oratorical Contest...(having) won first prize...best in the state". She imagines the whole school turning out to greet her in the parking lot, "just like in the assembly film". She is beside herself in anticipation, exclaiming to Leo, "I'm going to be popular!"
Stargirl does indeed go on to win the contest, but when she returns to Mica High with her hard won trophy, the silver plate, in hand, there is only one student on hand to greet her, the faithful Dori. Stargirl is stunned, and when her parents come to the car she "allow(s) herself to be led away", the silver plate falling from her lap onto the asphalt and ringing "like a dying bell". Stargirl's father picks it up, and Leo thinks at first that he will take it. Instead, with "a strange smile", he hands the plate to Leo (Chapter 26-30).
The silver plate represents the values of the majority, and is a tangible symbol of acceptance for Stargirl. She has tried, for Leo's sake, to be "normal", and to fit in with her peers at Mica High. Her attempts are unsuccessful; no one cares that she has won the silver plate, and she has sacrificed her individuality for nothing. Stargirl's father, apparently understanding the situation, gives the plate to Leo. The trophy means nothing to Stargirl; perhaps it will mean something to him.