A head teacher who banned a girl from wearing a facial veil told the High Court the wearing of the niqab "went against the school's ethos".
The school allows Muslim girls to wear scarves but not niqabs
She said her girls' school, in Buckinghamshire, promoted equality between men and women.
And she feared other Muslim girls would come under pressure to wear the niqab if it became part of school uniform.
The case was brought by the father of a 12-year-old girl whose elder sisters wore the niqab at the school.
He wants a judicial ruling on whether the school was right to ban his daughter from wearing the veil, which covers all of the face except for the eyes.
The school, the head teacher and the child in the case are not being named for legal reasons but the school is a high-performing girls' grammar.
Lawyers for the girl have already told the court the school behaved "irrationally" because it had previously allowed her three elder sisters to wear the niqab.
They argued that the school's ruling was breaching her right to freedom of "thought, conscience and religion" under Article 9 of the European Convention on Human Rights.
The school says the niqab does not comply with its uniform code.
The head also told the High Court the full face veil hampered communication. Teachers and pupils both needed to see facial reactions.
Teachers needed to be able to tell if a pupil was paying attention or distressed or enthusiastic.
We are very conscious of our duty to educate girls to regard themselves as equals to men
The head teacher, who took over at the school in September 2002, stated: "I believe that if the niqab becomes an accepted part of school uniform attire there may well be pressure brought to bear on other Muslim girls to wear one, either from the children's families or from their friends at school".
For a girl to conceal her face was contrary to the ethos of the school.
"As a girls' school for over 100 years, we are very conscious of our duty to educate girls to regard themselves as equals to men and to gain the self-confidence to live and work in British and international society on the same level as men," she said.
The head teacher also spoke of her duty to ensure the safety of her pupils.
"If a stranger is on site, then it is simple to approach and ask them their business.
"However, if pupils wore the niqab, then identifying those on site becomes difficult and it would not be beyond the realms of possibility for an unwelcome person wishing to move incognito to wear a niqab herself."
The judge will give his ruling at a later date.