Hmm I don't understand why teens are so obsessed with loosing weight, it's a very serious matter.
Just make sure you don't go under 113 pounds that is the normal healthy weight. anything around 100 or lower is very bad. Anything over 150 you can get down a few. I'm 115, possibly even 120 now and I have no problems with how I look, if I have some extra side fat no big deal, infact I could gain a few pounds.