In most cases, the teacher just wants to find out what your level of playing is. Play it at a tempo that you can tongue it comfortably. To work up your tonguing speed, use a metronome and set it at a tempo where your tongue can keep up with your fingers. Play it through a 10-20 times at that tempo, then increase the metronome to the next higher speed. Play it at that speed for 10-20 times. If you gradually increase the speed, you'll develop your tonguing speed. Some pieces may require double tonguing. That's where you tongue using T-K-T-K instead of T-T-T-T. If you haven't learned to double tongue yet, see these YouTube video lessons from Nina Perlove:
http://www.youtube.com/watch?v=9s3l_zBrJT0
http://www.youtube.com/watch?v=i3X_xI2Rto0
When you play it for the teacher, play it at the fastest speed that your tonguing (single or double) can keep up with your fingers and explain to the teacher that you can't tongue as fast as you can play the notes yet, but that you're just starting to learn double tonguing and ask for help - that's what the teacher is there for!
<Added>Here's a few more of Nina's lessons on tonguing and articulation:
http://www.youtube.com/watch?v=c7bA4ibn4OM
http://www.youtube.com/watch?v=8lqNKopV4B8
http://www.youtube.com/watch?v=s8fdUhAixxQ
This is her YouTube main page - lot's more good stuff there:
http://www.youtube.com/user/ninaflute
