I've not tried this yet (it just landed in my mailbox this morning) but now you can have up to 10 people collaborating live in a single Prezi.
You'd probably want participants — like your students — to be familiar with Prezi before you begin (it takes 30-45 minutes to get your head round using the Prezi interface), but the meeting option looks cool.
I like Prezi: if you're looking for an alternative to PowerPoint, it's excellent: it has far fewer options (which makes it much easier to learn) and it goes down great with kids, who are already bored with PowerPoint.
Here's an idea using Prezi, a fun alternative to PowerPoint, which makes minimal use of technology (a good thing), for practising conditionals, and would actually make for a fun piece of homework.
Here, I've taken an example produced on pieces of paper by learners on a recent technology training course, and I've created the presentation. Clearly, however, you want your learners to be doing the work. Who should use the technology, the teacher or the learners…? The latter, if you ask me — almost always.
Here's another cartoon creator (thanks, Kate!), this time from Chogger.com, with my first go with it in the screen capture, above.
You've got the sort of interactive tools you'd expect — text (including speech bubbles and captions), images (including drawing them and importing them), the ability to add new panels (3 shown above, the "active" one in white) and so on.
It isn't entirely intuitive if you're 50-plus like me, but to a kid familiar with technology this one will be a breeze.
You don't get the hundreds of ready made cartoon characters you do get at similar sites like ToonDoo, but I actually think that's a plus, as it leaves a lot more up to the user's imagination and creativity.
I particularly like the fact that you can import photos (in the screenshot, I've imported my SecondLife avatar). You (and I really mean your learners) can tell stories with speech bubbles in PowerPoint but I think young learners will love this one as an alternative.
My preference would still be for good old Word for writing collaborative stories in class [why?]– but, if you have a class blog, this looks like a fun piece of homework.
Personally, I have a preference for using good old Word, or even PowerPoint, or (best) a blog, for creative writing as I think that with some of the above your learners will end up spending more time on the technology and less time on the writing and the interaction in English (with the latter being what we're really trying to obtain in the language classroom)…
But I accept that, especially with younger learners, being able to animate things is probably more exciting, and hence more motivating and engaging and thus as likely as anything to produce learning.