Some school libraries are reporting big drops in the number of books teens are reading, and they're blaming social media and computer games for the fall.

They said the trend could be harming young people's ability to read long texts, use correct grammar and spelling, and succeed in life.

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Librarian at Marlborough Girls' College Colleen Shipley said the number of books issued at her school reached a high point of 13,420 in 2011.

But since then, issue numbers have plummeted by a third, or nearly 5000 books, to sit at 8942 last year.

Ms Shipley said it was partly because the English curriculum covered fewer novels than in the past, but the main culprit was social media.

"Some girls will read off their phones, but they're not reading books.

"They're reading off social media, they're reading fan fiction and if you have a snapshot look at that type of reading that they're doing, it's bad - the grammar's bad, the spelling's bad," she said.

"The big worry is that if they're not reading in print with long passages of reading, they lose the ability to concentrate for that length of time, and that is going to affect them if they want to go on to tertiary education."

Wellington High School librarian Jane Shallcrass said she too had noticed a fall in the number of books teens were reading.

She said research showed those who stopped reading for pleasure did not do as well in life as those who kept reading.

"That's not just in their literacy, but in all their school subjects and then in life after school.

"So whether they go into tertiary education or they don't, the reading habit means they'll be more successful in their job, they'll be more successful in their relationships."

School Library Association president Miriam Tuohy said book issues had been falling at her school, Palmerston North Girls' High School.

She said teens' reading habits had changed, but she was not sure that was a problem.

"I'm not one of those people that thinks it's a terrible, terrible thing that people are reading shorter texts per se.

"As long as these young people are reading something they enjoy, and they're not out of the habit of reading itself - that's where you don't want to end up."

And not all librarians have noticed a decline in book issues.

Dunedin's Kings High School librarian Bridget Schaumann said there had been no drop at her school.

"If anything, our stats have steadily risen over the last five years."

But she said librarians needed to be able to get in front of students in order to promote the books on their shelves and have a good budget, so they could buy what teens want to read.