The mother stood behind her shackled 15-year-old son and leaned toward the judge, trying to understand what was happening.The judge and public defender were volleying court lingo on this August day.Four months ago, the boy was convicted of aggravated unlawful use of a weapon, or as they said it, “agg UWW.”He was in the Cook County Juvenile Center for a “bring back” — a judicial evaluation after he served four months of his eight-month sentence in the Department of Juvenile Justice.If he …