In the past two posts I've discussed two different interns (Bill and John) I've worked with in test departments. One performed well and the other...not so much. Some of the lessons drawn from those experiences might apply to any intern, but I maintain that interning within a test group is a very specific experience that has specific requirements. Proceeding from this supposition, here are three things I think you need to do for an intern to succeed in a test environment.
Hire an intern with broad technical interests. Building and running test stations requires the engineer to be something of a jack of all trades. If the station is automated, you need to know software. You may be called upon to build some fixturing, so you may need to know a bit of mechanical engineering. If you need to debug a circuit, then an electronics background is handy. This can be fun stuff, but only if you _like_ doing different things. All Bill liked to do was program. He did that well, but not much else.
Scope the assignments. There are no courses that teach how to test - it is a very specific skill that is learned from experience. Bill often had little direction from the manager (who was busy looking for a new job at that time) and he floundered. I gave John detailed instructions, and he flourished.
Play to his strengths. This is a corollary to the first point. The intern may have interests in different fields, but he is still getting his degree in a specific discipline. Bill did a great job with the project that involved a lot of software. Since John is a EE major, I've given him tasks that minimize his programming. Choose projects that play to what the intern can reasonably do.