But parents criticised her comments about larger girls yesterday, warning it could make pupils feel more insecure about themselves.
One said: "This is a breeding ground for anorexia. No headteacher should comment on size and shape where teenage girls are concerned."
Another added: "So larger girls who experience bullying about their size should not expect the school to tackle the bullying, but should purchase looser clothes and hope the bullies are fooled into thinking they've lost weight?"
Yesterday, the head sent parents an email explaining her policy.
She said: "Ironically, for those girls who are not very slim, the tight clothing emphasises their heftiness and is unflattering. Thus this makes them prone to bullying.
"If everyone is covered up in slightly loose clothing, there is less bullying over body shape and size.
"Skinny-fit trousers and very short skirts are not flattering to some students and can leave them prone to mean comments." Writing to one parent, she said: "They should look demure and modest and not appear over-sexualised.
"The last thing we want is boys peering up girls' skirts while they are climbing the stairs."
But pupil Chloe Hirst claimed the policy was sexist after she was turned away at the gates.
Chloe, 16, from Milton Keynes, said: "The boys never get any hassle. It is disgusting. I've been wearing the same thing for two years, but I've never been sent home."
Defending the policy, Dr Jones claimed the matter had been "over-sensationalised", and said students and parents were warned that uniform checks would be in place at the school, which is rated as "good" by Ofsted, after Easter.
She added: "They are under 18 and still children. We don't think that very tight or revealing clothing is right in a mixed-gender school.
"It might seem old-fashioned concept, but we want girls to be modest to protect their self-respect."
However, Laura Bates, of the Everyday Sexism Project, said: "Girls' bodies and dress are often policed more severely than boys', and - especially when you start throwing around words like 'modest' and 'demure' - there is a sense of girls being judged in a way that isn't happening to their male peers."