I believe it's Isaac Stephens, killed in the battle of Chantilly or Ox Hill, Va.
Edit - You're correct that Stephens was killed in battle in 1862. Chantilly may not be as famous a battle as Antietam, but it was a significant battle.
According to Wikipedia, Stevens was born March 25, 1818 and graduated from the prep school Phillips Academy in 1833, so he did finish prep school at a young age. He was accepted to the USMA, from which he graduated in 1839. I don't have a precise date for when he was accepted to West Point. If he didn't graduate until 1839, it may be that a couple of years elapsed between his graduation from Phillips and the time of his acceptance at West Point. In any event, if he didn't graduate from Phillips until 1833, I would assume he wasn't accepted at West Point before his 14th birthday, which would have been March 25, 1832.
hoosier