Christian parents will not give up the fight to persuade a school to allow their daughter to wear a purity ring.

Lydia Playfoot, 15, was told by teachers at Millais School, Horsham, she would have to take lessons in isolation if she refused to remove the silver ring which symbolises her pledge to chastity until she gets married.

The school said it contravened rules on jewellery.

Lydia's parents Heather and Philip said they were disappointed with the way the school handled the matter as the ring was not a "frivolous piece of jewellery" but represented a serious promise to God.

Mrs Playfoot said: "This is not something we are going to let go of. We feel it's very important. Christians are often trodden over and not given the respect other religions are."

She said the school allowed Muslim pupils to wear head scarves and Sikhs to wear bangles in accordance with their religions and the rings should be recognised in the same way.

They are given to youngsters who pledge sexual abstinence after completing a controversial evangelical course called Silver Ring Thing. It started in America and was introduced here in 2004.

The Playfoots' church, Kings Church in Blackhorse Way, is the UK base for Silver Ring Thing and so far about 100 youngsters have made the pledge. About a dozen girls at the school have completed the course and want to wear purity rings.

Lydia said: "All my friends know what it is and they think it's a very positive thing, even if they don't want to do it themselves. I feel a bit singled out when teachers say I can't wear this ring but Muslims and Sikhs are allowed head scarves and bangles."

Mrs Playfoot said: "Purity rings go back a long time in the history of the church. Nuns wear them. It's part of our Christian faith about remaining sexually pure.

"At a time there are so many teenage pregnancies and epidemics of sexually-transmitted diseases we are disappointed the school has not been more supportive."

The Playfoots live in Beech Road with Lydia and sisters, Hannah, 21, Abigail, 19, and Miriam, 12, who all wear purity rings.

Lydia wrote to the school last month explaining she would no longer wear the ring in school time but felt it was unfair.

Her family intends to continue to lobby the school to change its policy.

Headteacher Leon Nettley said the school's sex education programme was sufficient to teach youngsters about the illegality of under-age sex.

He said: "The school is not convinced pupils' rights have been interfered with by the application of the uniform policy. The school has a clearly published uniform policy and sets high standards."