I know you do not want to use a travel agent - you don't have to USE one but they are a good source of free information. They will tell you what airlines leave your part of the country. They will give you some hints on when is the best time to fly too! Get the information and buy your tickets elsewhere if you want. With so many people involved in this trip a travel agency may be able to pull some strings and get you some good prices. Up to you!
Once you know what airlines fly from your nearest international airport then you can access those airline websites. All you have to do then is check the prices for various dates around your preferred departure date and see what a flight costs. You can even call up the airline travel agents and book in person. Start your holiday in Bangkok and end it there so you will be closer to the airport when you fly home.
My advice is to get a hotel near the Skytrain so you can use that overhead rail service to get around the city. If you are all kids in school then perhaps get a hotel in the Khao San Road area where there are loads of other kids around. No Skytrain in the KSR area!
If you are all a bunch of guys then see if you can fit Pattaya into your schedule. Lots of nightlife in that city and the hotels are cheaper than Bangkok. From Bangkok the bus ride is around 2 hours and pretty cheap. If there are women in your party then try to fit Chiang Mai into your schedule. In Chiang Mai you can take in some trekking too!
For tours in Thailand check out a local travel agency - there are hundreds of them around. These Thai travel agencies will have a menu selection of tours that you can look over. Tour company will pick you up at your hotel and return you there at the end of the trip. With such a big group you can ask for discounts off the normal price. First find out what the normal price is then mention that you have a huge group.
Wait until you get to Thailand to change your money over to Thai baht. In Thailand you have to use their currency. There are lots of bank money changers all over the main cities and at the airport. If you plan on using a credit or debit card make sure you tell your bank first so there is no problem getting withdrawals. You do not pay a separate departure tax at the airport it is included in the price of your ticket for almost a couple years.
Below are some helpful travel forums you can look over. If you need hotel recommendations make sure you mention how much per night you want to spend - don't say "cheap" or "middle priced" use a baht figure. To go to Thailand for only a week is not getting your money's worth from the airplane ticket. Once in Thailand the price of hotels are pretty cheap so what's another week really going to cost in the end?
http://www.tripadvisor.com/ShowForum-g29…
http://forum.virtualtourist.com/forum-14…
http://www.lonelyplanet.com/thorntree/fo…
http://www.travellerspoint.com/forum.cfm…
http://boards.bootsnall.com/asia-travel-…
Read up on how to take taxi cabs from the airport and how to take taxi cabs in the city. If any of your party are dopers be warned that in Thailand you can be in severe trouble if you get caught with drugs. Often the doper puts his dope in his buddies luggage so they take the hit. Before you fly home thoroughly inspect your luggage and keep an eye on it! Read up on scams and other problems you might encounter in Thailand.
Before the internet became popular "in the old days" people used to actually read travel guides that they purchase or check out from a library! You can visit out a library and get some idea of what is available from the travel section.
Google up "what to see and do in Thailand" and you will get many hits too!
And have a nice time in Thailand!