I thought this information was very helpful and informative. I am posting if for others to use as there seems to be some misconceptions of how growth of the spine typically occurs in adolescents.

I am posting a quote out of the Journal of Pediatric Orthopedics entitled Growth in Pediatric Orthopaedics by Alain Dimeglio, M.D.:

Journal of Pediatric Orthopaedics
21:549555 2001 Lippincott Williams & Wilkins, Inc., Philadelphia

During puberty, standing height increases by approximately
1 cm per month. At the onset of puberty, boys
have 14% ( 1%) of their remaining standing height to
grow. This is approximately 22.5 cm ( 1 cm) made up
of 13 cm in sitting height and 9.5 cm in subischial leg
length. Girls have 12% ( 1%) of their standing height to
grow. This is approximately 20.5 cm ( 1 cm) made up
of 12 cm in sitting height and 8.5 cm in subischial leg
length (12,3436) (Figs. 2,3).

Growth rate peaks during puberty between 13 and 15
years of bone age in boys and 11 and 13 years of bone
age in girls. By the time girls and boys pass bone ages of
13 and 15, respectively, lower limb growth comes virtually
to a standstill, with all remaining growth (4.5 cm)
taking place in sitting height

I can't post the whole article because it is a PDF file and I can't find a link anymore to the article...who knows where I picked it up. But I think this clearly shows that what I have seen in ballet students, i.e. in the ninth grade some of the ballet students suddenly look longer in the torso relative to their legs.

I wish I could post the whole file because it's extremely interesting and has a couple of great charts. If you can get hold of it somehow, it is a good find.