PHILADELPHIA — In 1931, an all-white jury took just four hours to convict Alexander McClay Williams, a Black teenager, in the stabbing death of a matron at the Glen Mills School for Boys in Delaware County, Pennsylvania. Five months later, Williams, 16, was executed, becoming the youngest person in Pennsylvania history to be put to death. His family spent decades trying to prove his innocence, and this week — with help from the great-grandson of the lawyer who represented him at trial — Williams was posthumously vindicated. A Delaware County judge overturned his conviction for a crime prosecut…