hey, i'm 14 as well and i also ended up gettin' horrible smiles in my photos.
but the last photo i got looks good coz i was natural and smiling ear to ear all the way!!
i think if you try to remain a little bit less self consious during photo sessions,it may help coz worrying about how U you look reflects in your face,and that's the main problem which makes smiles appear weird. try being natural and smiley all the time ,it's surely gonna help!!
P.S. sorry if you have trouble understandin' my english,coz i'm not really good at it!