Ethnic tensions, intruders and the enrolment of disaffected people are all contributory factors, say NUS officials.

At Redbridge College, east London, a gang of six students has been caught mugging other students for their cash and mobile phones. But only one of the six has been expelled. In another incident, a gang from outside the college attacked a student on the premises, injuring his hand.

Police say neither of the incidents, which happened towards the end of last term, had racial overtones. However, the college has closed the student common room which, says college principal Dr Tony McGrath, “was being used by only one ethnic group”.

At Manchester College of Arts and Technology (MANCAT), an intruder who refused to stop using a mobile phone in the library, turned on a member of staff as she tried to escort him from the premises. She was threatened on a staircase and fell down several steps injuring her back.

At Bury College, a security officer was injured when he tried to break up a fight between students. He slipped and dislocated his knee. Those involved are currently going through the disciplinary procedure.

Colleges are responding in various ways. Some have increased security, using guards and closed-circuit television, while others try to move away from the spectre of stern men in uniform by appointing extra youth officers and training staff to cope.

Redbridge has installed more cameras inside and out, and introduced a sophisticated portable radio system and extra lighting in the car park.

Increasingly, security is a worry for students at the Colchester Institute, who requested CCTV to be installed in the common room more than three years ago. “It cost pound;6,000 - 7,000, but it has paid for itself several times over in reducing vandalism,” said liaison officer Clive Wilson. “Through one ofthe videos, we recently managed to catch someone in the act of stealing a purse.”

MANCAT has a different emphasis. “We had a massive bill for guards contracted through a third party,” said assistant principal Jack Carney. “We felt this was dead money - they were only superficial cover.” The college has scrapped the requirement for students to produce an identity card on arrival. “It was causing queues, which got people frustrated,” he said. “We had to turn away those who didn’t have cards.”

Mr Carney claims staff vigilance and initiatives taken by 12 youth workers, has cut unruly incidents. Yet, in common with many colleges, it is reluctant to kick out trouble-makers for good. “Some we have excluded, then reviewed the situation and allowed them back. We need constantly to look at staff behaviour as well as that of students,” he said. But NUS officers fear the overriding desire not to banish miscreants can store up trouble. “Procedures on discipline and harassment vary,” said Bill Freeman, NUS eastern regional officer. “A national standard might be worth looking at.”

At West Herts, with campuses in Watford and Hatfield, regular meetings of a security forum, involve students, management, outside security, the council and the police. All students, including part-timers, have identity cards, and a quick-response communications system alerts people to problems on any campus.“Staff joining from other colleges brought examples of good practices,” said college spokeswoman Jacky Bennett.

Redbridge College youth worker, Alan Kipping, has been in the job 10 years and is responsible for organising extra-curricular activities. “Students who come here have to work more and more - many at least two days a week,” he said.“Involvement with student union activities has fallen because they don’t have the time. When the common room was taken away, there wasn’t enough student clout to put up a fight. I think there is a problem of disaffected youth - there is no spiritual guidance for them any more.”