Reading this article, it is clear that Pinterest, a social networking service would be of use for mature students.
The key for school teachers is this section:
"... Pinterest does not permit those aged 13 or under to use the site, Christina DiMicelli, a technology integration specialist at Hampstead Academy in Hampstead, N.H., does not feel comfortable in allowing students at the K-8 school to use it in class.
“There is a lot of unmonitored content on it,” she told the Daily Dot. “It is too easy to come across images of nudes, etc. I am hoping that Pinterest comes up with an education account for teachers with managed student accounts included.”"
The message here is that teachers need to read the Terms and Privacy statements for all sites used with students and be clear on age eligibility requirements.
Statements such as "I didn't know, didn't see it, it is a law of another country etc " do not constitute an effective approach to risk assessment.