The 15,000 members of the Exclusive Brethren follow a rigid code of conduct based on a strict interpretation of the Bible. They shun the outside world and have no contact with other Christian organisations.
The Brethren cannot eat with non-members, are forbidden from living in homes adjoining families who are not in the sect and school-leavers cannot attend university - which they regard as too worldly.
Sue Collins, the former head of a Brethren school at the centre of the unfair dismissal case this week (see main story) told The TES: "I wasn't allowed to eat with the pupils. If we went on an outing, there would always be adult members of the Brethren with us - they would eat their lunch in one place and I would have to go somewhere else to have mine."
Other sect rules include the stipulation that men should be clean-shaven, keep hair short, and not wear ties, and women should leave hair uncut and wear blue or white headscarves.
Mrs Collins, who has taught in the private and state sector for more than 25 years, said the curriculum in Brethren schools had to be altered to fit in with its closed view of the world.
"I was criticised for talking about the Nazi view of homosexuals during the Second World War - even for using the word homosexual," she said after the tribunal.
"When you teach the Middle Ages you are supposed to talk as little as possible about religion, and you must never talk about evolution. They are even trying to stop them learning biology."
In 2002, the Brethren came under the leadership of Bruce Hale, known like his predecessors as "the Elect Vessel". He adopted a new policy towards those who have left the sect, seeking to bring them back rather than shunning them.
Contact with families is now likely to be banned only if they are actively hostile to the Brethren. But members are still barred from joining trade unions, visiting other churches, sharing a driveway with an outsider or even sharing private drainage facilities, in case it brings them into contact with sinful people.