And girls from some ethnic minorities and lower socioeconomic groups are more likely to be overweight or obese, putting them at even more danger of long-term health problems, say the authors.
Previous studies have shown that adolescence is a key time, since excess weight during teenage years pre-disposes adults to continued weight problems - with all the associated health risks.
But today's study, tracking 5863 children as they developed into young adults, shows that the problems are established before teenage years - since those with excess weight by the age of 11 continued with it during adolescence.
Researchers looked at annual measurements of weight, height, Body Mass Index (BMI) and waist circumferences of children aged between 11/12 and 16/17 throughout 36 schools across South London, giving them a broad ethnic and social mix.
They found that overall, girls had higher rates of excess weight problems than boys. Black girls had particularly high levels, with an average of 38% being overweight or obese over the study period, compared to 28% for white girls or 20% for Asian girls.
For boys, however, ethnicity made little difference to excess weight levels.
The findings were less clear cut for economic status. 35% of the most deprived girls were overweight or obese compared with 28% of other girls, but other economic categories showed less consistency.
"Children who are obese when they enter secondary school will very likely leave it obese," say the authors. More monitoring is crucial if rising tides of obesity are to be tackled effectively, they conclude.