Richard Sykes, a barrister with the education law firm Mills amp; Reeve, said a college could find itself in "uncharted territory" if an adult sex offender struck in a college.
"The general position is that one doesn't have a liability for the actions of third parties," he said.
"If the college has in some way assumed a specific duty of care in the context of its students, someone might bring a claim. But we would be in largely uncharted territory.
"It would turn on the facts of the case: the extent to which the college knew of convictions, and the extent to which the college was able to take steps against it."
Colleges have a dilemma in choosing between conflicting duties of protecting children and rehabilitating offenders, he said.
"There are competing rights involved here. An individual would have rights under the Rehabilitation of Offenders Act, and a college has a general duty to conduct itself in a way that protects the welfare of children who are learners in further education. But there's no general law that colleges have to run background checks on their students."
Even if colleges do ask adult students to disclose previous offences, after a maximum of 10 years former offenders do not have to divulge to them, as long as the sentence was less than two-and-a-half years.
Criminal Records Bureau checks would reveal even these convictions, but they can only be used in special cases - such as for students on childcare courses. Mr Sykes said colleges were in a difficult situation with former offenders, given their involvement in prison education and educating former criminals.
"One of the issues at the heart of the treatment of offenders is upskilling them. Colleges have an absolutely central role in the delivery of skills to a very, very wide and varied population."
Government guidance on protecting children in colleges concentrates on groups who must have criminal records checks because they have regular, unsupervised access to children, from teachers to governors and volunteers. But because colleges are self-governing, decisions about whether former offenders enrolling poses a risk is a decision for college management.