I would also recommend Trowunna. It's not huge as zoos go, but the fact that they so carefully recreate natural environments for the animals is to be admired. You won't need very long here. No more than 2 hours, unless you're just enamoured with the Tassie Devils like we were.
We also did the Seahorse World and nextdoor Platypus House in Beauty Point. After having been platypus spotting in the wild, seeing platypus in tanks was a bit of a letdown. The part we enjoyed was meeting a couple of echidnas up close in their very own room. The Seahorse World was neat, but it was one of those experiences that I had hyped up in my mind and afterward felt the time (& money) could have been better spent elsewhere.
If you want a platypus guided tour, I'd suggest the one in Latrobe. At a $10 donation/fee it's cheaper than the Playtpus House (was $15 when we were there in 06). The downside is the time limit of dawn- or dusk-only tours.
We regretted not having enough time to see penguins. I guess it depends on what animals your teenagers are interested in as to what to do. We love any and all animals, so we really enjoyed the experiences Tassie had to offer.