Before adolescence average body fat is 18% in both boys and girls. However, after age 13 boys drop 2-3% in body fat and girls gain 8-10% in body fat. This is normal and due to the different needs of men's and women's bodies as a result of sexual development. I emphasize that these are averages. Each of us have different metabolisms based on our genetics. We store energy at different rates and in different areas on the body. There is no optimal body fat percentage.

Concerned students may want to consult a doctor or nutritionist. Here are two pieces of information that I found on the web: During the teenage years through most of adulthood, we are healthiest when our bodies are at their slimmest, this is when being "thin" is a good idea. However, this does not mean that if your thighs stick out a little too much or your tummy doesn't have that washboard look that you are fat! You should be thin in terms of your body fat percentage (the percentage of your body that is made up of fat). Did you know that people who are anorexic are actually fat? Yep,it's true. Anorexics usually eat almost nothing and exercise for hours each day, which puts the body into starvation mode. The body starts to break down muscle for energy, so all that is left in the body is fat. In this case, a woman might weigh 80 lbs., but she is full of fat. She is a "fat" skinny person. This extreme example shows why it is very important to continue to eat enough calories when you are dieting, if you want to lose weight effectively.

Requirements for energy and nutrients vary through life depending on a persons gender, age and activity level. Estimated Average Requirements(EARs) exist for energy and nutrients according to these different factors. The average energy intake for teenage boys aged between 11-14 was 1968 kcal per day (89% of the EAR). For boys aged 15-18 years average daily energy intake was 2285 kcal (83% of the EAR). For girls aged 11-14 years the daily average energy intake was 1672 kcal (89% EAR) and for 15-18 years, average daily energy intake was 1622 kcal (77% EAR). Even though each age group did not meet their EAR for energy, i.e. did not achieve 100% EAR, there was no evidence that their average energy intake was insufficient (as a group they were not underweight), instead they were inactive so did not require as much energy as estimated to be needed by teenagers of that age/gender.

I'm glad you're focusing on long-term health.The American Council on Exercise gives the %Body fat range for adult males and females as follows:

Athletes: 14-24 F, 6-13 M
Fit: 21-24 F, 14-17 M
Acceptable: 25-31 F, 18-25 M
Obese: over 32 F, over 25 M
Less than 14 for females or 6 for males is not healthy.

The CDC (Center for Disease Control) has charts on Body Mass Index available for ages 2-20 at this site: here It uses age, height, and weight to calculate BMI and explains how to interpret it.