What vitamin would help my teenager who has a hard time with focus and short term memory?

Hard to say: People perform at their best when they are well rested, have a balanced nutrition,are not ill and work in a familiar setting. Unless the kid has a vitamin deficiency, supplements would not help. Teens need a minimum of 9 hours/d with increases if studying hard, playing sports or in a growth spurt.Fix what you can and get a checkup if this does not improve.