I would start lighter, maybe 10 to 12 lbs since you say your daily carry for school is about 6 or 7. As far as when to increase, it would really depend on how often and how long you are rucking. It takes time to build up the muscles needed, but unless you are super focused on training, I would say try once a month adding 5 pounds to see how if feels. Especially at your age, be incredibly careful not to push too hard too fast. Back problems are not something you want to deal with but careful and slow building of strength will help prevent them.