On the basis that you have addressed the use of the film in terms of copyright and any other relevant intellectual property rights, we shall not comment on that area.
The showing of a film rated 18 to students who are under 18 raises issues of risk and the level of risk the college is willing to take. Institutions have a duty of care to students, and this means that the institution must take the steps a reasonable person would take to prevent harm or damage. It could be held that showing an 18 certificate film to under 18s breaches this duty of care, and you would be liable for any harm or damage that occurred. JISC Legal has relevant materials in the
e-safety section of our website, in particular the
Safeguarding - Meeting your e-Safety Duties (03/03/2010) webcast where the law and the extent of duty are discussed. It should be noted that in addition to the legal duties, quality inspection bodies often rate the provision of a safe learning environment as part of their assessment, and the showing of 18 rated films to under 18s may be judged by them to be inappropriate.
The British Board of Film Classification has an FAQ which is relevant, and suggests that such showing an 18 rated film to an audience younger than that age is not appropriate - (1)
Can a school show videos/DVDs of a particular category to children below that age? This is the type of scenario which may have the effect of damaging the college’s reputation in the event that the viewing of an 18 certificate film by students under that age results in parental complaints and such complaints attract the attention of the local press. These are all risks the college needs to consider when assessing how to proceed.
One option to reduce the level of risk which is mentioned in the BBFC FAQ would be to seek written consent from the parents of under 18 students. Alternatively you may be able to use a different film which fulfils your purpose but is not rated 18.