Ok, I wouldn't say the ideal should be an specific breast size, but to be in proportion...i.e. waist to hip ratio and breast to hip ratio, and healthy.
You'll still be growing so don't worry if you're not that yet. I was totally pearshaped until about the age of 16 - I was a 32AA with 36 hips. Now I am 36 25 34....I grew into myself.
However, I can remember when I was 14 and I went to an all girls school. So I would say that the average then would be a 34B, but by no means use this as a guideline for yourself.
Hope this helps.