Dear dstang,
Either from your query or from your profile it is not clear as to where you are based. Hence, it is difficult to say what the law of the land is. However, Madhu is an expert in law and he has stated the position. Hence, I will raise a few questions before giving a weblink about the law in USA that can be used as a guide only.
Where are you based and how large is the firm?
What is the objective of the internship: give an opportunity for the student to study the organisation and it's practices or help the student to see how theory learnt in the college applies in practice or is it to assess the student for an offer of a job? For example, as a student at IISc, Bangalore, we had to undergo industrial training every summer. We were only expected to observe and not contribute. Most students undergoing a sandwich course get paid internship in their 3rd year of a 4-year course and they are expected to actively contribute.
Dinesh Divekar has given you a sound reason why one should pay for interns: to attract better quality students. Willy Franzen in USA has written a good article on "Are unpaid internships Illegal?" at
Are Unpaid Internships Illegal? | One Day, One Internship
It lists 6 criteria that should met if an intern is not going to be paid. I suggest that you kindly read and see if it is of any help.
Also please see
Buena Vista University - Paid vs. Unpaid <link updated to site home>
Both found at
Google
Have a nice day.
Simhan
A retired academic in the UK