I was sorry to read Shereen Pandit's article in The TES 9th February. Her views about libraries are disappointing and I would suggest that she discusses the book buying policy with Barnet Library Service, as I cannot answer for them specifically.
I can however speak as a very experienced school librarian, having worked for over twenty years in a wide range of schools including those in inner London, in Derbyshire, in Cambridgeshire and currently in Hertfordshire.
I have worked tirelessly as have many other school librarians all over the country to promote fine children's writers. There are some circumstances in which the Sweet Valleys, Point Horrors etc are to be encouraged, perhaps for children who will not read anything else at that time, but in fact my experience is that children can be enticed to try other writers through enthusiasm for love of a good story.
Most school librarians that I know provide a very wide range and mixture of different writers and often have reading award schemes, quizzes, competitions, book clubs, book fairs etc.
Our school, as do many others, works closely with the English department in providing a lists of recommended books (not a Sweet Valley in sight!); students can earn bronze, silver and gold certificates throughout the year, which is proving an excellent way to encourage wide reading of contemporary and classic children's fiction.
I can understand Ms Pandit's disappointment if she cannot find excellent writers on the shelves of her local children's library, but I am certain that if she requests titles or even discusses the issue with the children's librarian the response will be helpful and enthusiastic.