http://www.dailymail.co.uk/pages/live/articles/news/news.html?in_article_id=428488&in_page_id=1770
It actually says "lapel badge" and mentions that she just recently got the necklace as a present from her father. In other words, they knew about the ban before she got the necklace.
Also, this school is oversubscribed so I am sure they knew about the rules before she got in. OTOH, the inspection reports give the school unsatisfactory ratings in religious studies.
Thanks for the link.