There are five essential nutrients for effective brain function, and this includes memory. Many teenagers are low in iodine, as can be said about the general population. It is readily found in some seafood but if you do not eat seafood then you must obtain it from another source.
Decades ago the government placed iodine in table salt so that inland populations may have a non-seafood source of the nutrient. If your family eats salt then it may be beneficial to purchase iodised salt. Other nutrients are omega-3 (from oily fish), iron (meat), zinc (almonds) and the B vitamins.
Two herbs gaining respect for their ability to aid memory are Ginkgo biloba and gotu kola. Ginkgo increases blood flow and fluidity to the brain and improved circulation aides brain function.
The quality of food is so important. We do no good for our children when we give into their tantrums for low nutrient junk food. Be parents to your children now and their friends when they grow up.