Get the NF! I don't think the Alma might be the best choice for a 15 year-old-girl!
I'm 19 so I'm not that older than you and I can tell you that sometimes I did "steal" my sister's alma and it does look not right for my age, but hey maybe it's just me! I tend to dress more casual or dressy, probably not the right outfit for the alma!
Btwwww I know you've mentioned big bags.. but why don'y you consider the eva?
I know it's a clutch but I think it should be perfect for your age (:
I think the NF would totally suit you (not just looks-for-age wise, but in terms of practicality too- It'd be your best friend throughout school and beyond to uni ) I agree with Monsouris that you'll have plenty of time to grow into the Alma!
p/s: I'm 5'2" and the NF MM looks perfect on me