Before he ever set foot on the Lehigh University campus in Pennsylvania this autumn, one first-year student's reputation had preceded him.

First, on the university's Facebook page, the student posted his vehement disapproval of a required summer-reading selection. Then he shared on social media the bad things he said he had heard about the dormitory where he would live.

The student was "mortified" when the university responded to his comments not with an offer of a different room or reading list, but with a warning that his online behaviour threatened to derail his university career before it had even started, said Allison Ragon, Lehigh's assistant dean of students and director of first-year experience.

Explaining the university's position, Ms Ragon said: "We want you to share your opinion. We don't want to squash you having a view. But we want you to use this media in the right way."

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Themselves mortified by widely publicised incidents of misconduct by students on social media - perhaps most notoriously the suicide two years ago of a student at Rutgers, the State University of New Jersey, after his roommate secretly recorded him in a sexual encounter with another man and told his followers about it on Twitter - US universities are now cracking down on poor cyber-behaviour.

In such an environment, "we almost didn't have a choice but to jump in", Ms Ragon said.

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Although protections for free speech make it difficult to regulate expression on social media, universities are applying existing codes of conduct to students when they are online, as well as requiring them to undergo "netiquette" training.

"It's such a hard medium to manage," Ms Ragon said. "We don't want to look at things in a punitive way. We want to have the leeway to have the conversation about: 'Why did you do this?'"

To retweet or retreat?

Like many institutions, Lehigh has begun to teach good cyber-behaviour as part of its orientation programme for new students. This includes explaining how misusing social media can come back to haunt you.

"You're a post away from screwing up every time," said Matt Hames, manager of media communications at Colgate University in New York State. "But that doesn't change the fact that people are going to use [social media]. So instead of being afraid of it, it's better to educate people about how to use it."

This autumn, Colgate began to offer training in how to use Facebook and Twitter responsibly to student athletes, who have been singled out for attention because they often have many online followers and because the consequences of their bad behaviour can be extreme.

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"We've had a couple of times when we've had to track down...athletes and ask them why they [had posted something]," Mr Hames said.

With such interventions, universities aim to protect not only students but also themselves - their reputations can suffer when student misbehaviour online goes viral.

The University of Kentucky goes as far as monitoring its student athletes' social-media accounts under a contract with a service that alerts coaches whenever players post or tweet any of 400 "trigger" terms, including "porn", "doobie" and "beer pong". Until the monitoring was disclosed, "Muslim" and "Arab" were also on the list, but they have now been removed.

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The sensitivity of the issue has been reinforced in Maryland and in California, where state legislatures have banned universities from forcing students to disclose their social-media user names, calling that a violation of privacy.

"It's extremely complicated, and there are so many nuances relating to free expression and free speech," said Sameer Hinduja, co-director of the Cyberbullying Research Center. "Courts and legislatures are still struggling with these issues."

In encouraging responsible use, many universities have chosen to focus on telling students not how social media can hurt them but rather how it can help.

If you let them know that Facebook and Twitter are "potential marketing tools" to help land jobs, Mr Hames said, "they'll think of social media in a different way. We hope that by showing them what it can do, they will actually think: 'Does posting this picture of my drunk roommate help me with my career?'"

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