Rt Rev Rachel Treweek, the first female diocesan bishop, spoke to a group of seven girls and six boys at All Saints Academy in Cheltenham.
Every girl told her they weren't happy with their appearance, compared to only one boy who said the same.
Bishop Rachel's visit comes after a report in August from the Children's Society, which found one in seven 10 to 15-year-old girls were unhappy with their lives, and one in three weren't satisfied with their looks.
In comparison one in nine boys were unhappy with their lives, while one in five didn't like how they looked.
One girl told Bishop Rachel: "I wish I was prettier and maybe a bit thinner and then I would feel more confident.
"The boys can be really mean and call you fat, but then I think that's because they have their own insecurities."
Another said: "It's like people feel they always need to be fixed and that being yourself is never going to be good enough.
"It is what is inside that counts, but the reality is that physical appearance is important and I don't see how these pressures are going to disappear."
Bishop Rachel said: "As a follower of Jesus Christ, I want young people to discover who they are and who they have been created to be, by God, and to know that they are loved and valued.
"It's important to think about how we talk to each other and to ensure that what we say gives confidence to others."