I suggest doing none of the exercises stated apart from those of squat and deads as stated by MXD
if you are going to train the core for some unforseen reason as a separate section of the body what ever you do one way you do the other way
This means training the posterior chain and ES especially.
I appreciate as a 15 year old there is a lot of emphasis placed on body image these days but simply doing crunches, raises etc etc is not ideal for posture development.
Whilst your target audience of 12-14 year old girls like 'skinny kids with 1/2/3/4/5/6 packs' look at the big picture and see this as an opportunity to build a strong core through big movements
You dont have to go heavy but squats and deads (normal and SLDL) will produce not only good ab development but a strong core
If you are hell bent on simply working your abs then do some dorsal raises for your lower back to have some postural balance before you end up looking like the hunched over eagle like creature that appears on so many faceparty profile pictures.
exercises for lower back here
http://www.exrx.net/Lists/Directory.html
whilst simple deads/squats/bench/chins/mil press exercises can be found on this site.
If you are not going to weight train to get visible abs, which i believe to be your goal, your BF must be low as the guys have stated and when muscle mass is low and BF is low it simply looks like you need a feed or you are trying to audition for the local marathon runners team. Neither look is ideal just to show your 3 pack off on the 2 sunny days we get here in the UK each year.
i personally love SLDLs with Dbs for both ES and RA deveolpment
In laymans that simply means doing the exercise above combined with low body fat will give good core strength