Hi just-a-girl. So you need somewhere safe which caters for large groups and is suitable for relatively inexperienced divers?
I would suggest the Great Barrier Reef in Australia.
A good place to base yourselves would be Airlie Beach, which is about an hour from the outer reef by high-speed cataraman. Airlie is a real party town and you'll meet heaps of back-packers from Europe and the UK/Ireland.
At that time of the year the daytime temperatures in Northern Queensland are usually 22-28 degrees C, and water would be about 22 degrees. You'd need a 3-5 mm wetsuit or dive skin.
The diving is well regulated and caters for all of the better known agencies, including PADI, NAUI, and FAUI.
The reef is an endless delight, with coral and great fish-life.
If you are feeling a bit more adventurous, you could head north to Townsville (4 hour drive) and dive the Yongala, a wreck which was lost at about the beginning of the 20 th century. It's totally encrusted but you can make out the hull and a few features. The main attraction is fish-life. It's quite magnificent. There are resident groupers living under the bow. One is called V-dub, because she's about the size of a volkswagen. (V-dub's mate is notorious for trying to swallow a Swedish diver's head on one occasion. No harm done, though it scared the carp out of him.)
Yongala lies in about 30 metres and is not a difficult dive, though occasionally there's a bit of current running. It can be done as a day trip, but the best way to do it would be on one of the larger live-aboards. Expect 10 dives over 3 days. (2 nights on board.)
No special vaccinations are required, there is no malaria, and the Australian dollar is weak in comparison to the Euro, so your money would go a long way.
If you're interested, I'd suggest do a Google search on "Airlie Beach", "Scuba dive Queensland", "Yongala wreck" and "Great Barrier Reef live-aboard".
There's great diving to be had also in Vanuatua (formerly New Hebrides) in the South Pacific, but the dive operaters might have difficullty catering for such a large group. Suggest take a look at
http://www.nautilus.com.vu/scuba.htm
If you need any more info, feel free to get back to me.