The plush red seats at the Wednesday matinee of “Hamilton” in Denver were filled with 2,700 teenagers who’d spent weeks studying a special curriculum about the hip-hop musical’s namesake, Alexander Hamilton, and the other Founding Fathers. Even though the show’s four-week Denver run had been sold out for months, the teenagers were seeing it for free. Some of them had dressed for the occasion in high-heeled boots and three-piece suits.