Do interpreting teachers need to be practicing interpreters?

I'd say it is just as important for an interpreting teacher to be a practicing interpreter as it is for a swimming instructor to be able to swim. How can you teach something you have never practiced yourself? However not all interpreters are good teachers. You need to be a practicing interpreter AND a trained trainer. In addition, I think any consecutive class should start with a demonstration done by the teacher. The teacher will establish his credibility and the student will know what is expected of him.

As an interpreting student I have taken all my interpreting classes from practicing interpreters and it was a real pleasure. I think it is essential for an instructor to be able to offer practical advice and share his/her interpreting stories to motivate and guide students. Besides this helps instructors to establish and maintain their credibility. That being said, I also believe that it is important to have instructors that are not professional interpreters but focus on interpreting related research. These instructors may help spot potential pitfalls and problems that may not be very obvious to practicing interpreters.

PS. One problem related to having practicing interpreters as instructors is that they may have to skip or postpone classes every now and then due to unexpected meetings, events or organizational issues.

I don't think they need to be practising interpreters as such, but I do think they need to have previously worked as interpreters, prior to their teaching position. It is quite understandable that someone who has experience in interpreting goes for a career change or just finds they cannot combine full-time teaching with interpreting. When you are taught by interpreting lecturers and some have and some do not have interpreting experience, you can definitely tell the difference. If you ask a lecturer how they would have interpreted a certain text and they cannot give you an answer, then that is not good. On the other hand, the 'proper' interpreters can model things for students and will often give practical advice.

I suppose individual qualifications are slightly academic when it comes to interpreter training, it's the collegiate body that counts. In my view diversity is a good quality indicator to look out for when picking your school. At the end of the day I think interpreter training is not only about the development of skills but also about exposure to (different) role models. The more the merrier - a healthy mix of e.g. translators and interpreters should help students stay grounded.

Active interpreters will be able to bring to their students all sorts of interesting examples (subject to the respect for our principle of confidentiality) which are related to the here and now. New terminology, difficulties, terminology are in a constant flux and a professional interpreter who is active will bring a wealth of information to his/her students. Nowadays googling is easy and a prospective student may determine whether his/her future instructors are active interpreters or not.

Some if not most should indeed be practising interpreters (not all though, a well rounded curriculum should call upon other competences too) preferably with pedagogical credentials as well; the old chestnut "those who can do, those who can't teach" shouldn't apply here...and where it does the results will speak for themselves, I'm afraid! Schools should make teachers's cv's available to students and if they aren't a quick search around professional associations' sites and/or google will soon clarify that, if questioning them is unwise or unproductive :-).