How much water should a 12 yr old drink every day?

Daily fluid intake: General recommendation is 2 litres (68 fl oz or 2+ quarts) of fluid intake per day as the body eliminates this quantity of fluid every day through the skin, lungs, bladder and intestines. Children may not drink enough because their feeling of thirst is not developed enough. They also have a higher fluid requirement if it's hot or they're very active. High altitudes & cold climates also req more.

Don't sweat it: As long as the kid has normal kidneys and makes urine every 6 hours or so, there is no reason to have a specific target.There is fluid in vegetables, fruits and other sources that fill fluid needs. Normal pee/soft poo all verify healthy fluid intake.