What sorts of photographs are these? Are these group shots or individual shots?
Generally, you will want to ask whomever is pictured in the photo if you can display their photo online (being specific as to
where online you wish to display the photo). The permission does not have to be in writing, but it helps to have it in writing, and, because of your ages, it really should be a parent who gives the okay to the display of the photo.
Not all photos displayed online will require permission from those pictured, but you apparently have done some research and understand that, for commercial purposes and for for-profit purposes, permission is required.
Even when permission is not required, however, whenever you are handling another person's name or image, you must be careful with it. Getting permission prior to the display of their image online is the smart thing to do. It can avoid problems in the future.