Helen Fraser, chief executive of the Girls' Day School Trust has said that 'fit' should not be used to describe attractiveness.
Mrs Fraser also said an obsession with looks is preventing girls from playing a proper part in sport.
In a speech to the GDST annual conference, she said: "I have been in our schools when a class of teenage girls is returning from a sports lesson, pink in the face, tousled, throwing themselves down on the floor – with never a moment of 'am I having a bad hair day', or 'does my bum look big in this?'.
"Really great inclusive sport takes girls away from all those unhelpful thoughts of bodily perfection and focuses them on the team and the goals."
She also criticised the use of the word 'fit' to mean 'fanciable'.
She added: "What on earth has happened to the word fit, which used to mean that you could run a few miles without collapsing but now just means 'fanciable'?
"I think sport and exercise is one of the ways in which women can reclaim their bodies from the kind of obsessions of the tabloid press and celebrity magazines.
"It is not about being fat or thin, it is about being fit. It is not about how you look, but about what you do."
She said the GDST – which runs 24 private and two state schools – has created a culture of 'sport for all' by attempting to tailor PE towards all girls.
In addition to competitive team sports, some schools have introduced disciplines such as zumba, yoga, pilates, rock climbing, tai chi, rowing, golf, taekwondo and trampolining to appeal to different pupils.
But Mrs Fraser insists girls also need role models with a strong sporting past to emphasise the importance of staying healthy and physically active.
She cited the examples of successful women with a sporting heritage, including Christine Lagarde, head of the International Monetary Fund, who was a member of the French national synchronised swimming team, and Condoleezza Rice, the former US Secretary of State, who is an accomplished golfer.
She said: "It is important to have cultural representations of women and girls who are physically powerful, and fast and strong."