I thought religion was kept seperate from school nowdays anyway? Surely this debate must be a religious subject and therefore a bit risky for a school to ask for anyway?
Well if you want to do well just debate the best reasons you can. Teachers aren't looking for a winning team but for presentation, research and effort. Do the best you can and you should be graded accordingly. Just hope your opposition haven't done their research in some areas and you can get one up on them. There are creationist claims that sound good on the surface, but with research often fall through, but of course in a debate situation there is no time to research counters, so you could get in some points that way.
The best ones to use are the ones that require heaps of scientific knowledge to put down. So start with evolution takes so long you can't see it to test it. Man can't create life, just amino acids which aren't life. In theory you would need two creatures to evolve together for breeding, if one frog left the sea it would die alone. Mention freaky animals like the platypus, what the hell did that thing evolve from? And mention the stranger fossils, such as the ones that point to dinosaurs evolving into whales. Label evolution as only theory, that even the scientists admit has gaps (quote darwin if possible), whereas creationism has been an accepted idea for thousands of years.
You can in theory argue almost any point, just got to think how hard will this point be to refute?