That would be a great feature but unfortunately it's not
available at this time. We would need to think pretty hard about
how we would present results in that case. It sounds more like
you're wanting to run two separate surveys but pre-qualify your
participants. You might be able to use SurveyMonkey or Wufoo to
perform the pre-qualification question, and then redirect to one of
two Treejack surveys based on the result.
Alternatively, you could of course just send out both links, and
explain in your invitation email that participants should take
survey A or B based on whether they are a teacher or student.
Sorry there isn't a smoother solution to that problem right