Everyone is right, find a good CRUISE ONLY travel agent.
Carnival requires an adult over 25 in each cabin, so if 12 are going then FOUR adults have to go with a ratio of 3to1 student/adult. You'll need to get four cabins quad cabins to accommodate everyone.
The big problem is going to be who is going to pay what. This is where the fights start. It has to be fair and equitable for everyone. Therefore, using and example, if the cabin cost for four is $1000 then everyone pays $250, even if you are the one stuck sleeping in the upper berth (and that should be done fair and equitable between the students to each take a turn on the upper berth, UNLESS you have anyone who is a little fluffy and over 220lbs, then you don't want them sleeping in the upper berth as they have a weight limit.
You may also want to think about more chaperones because the drinking age in Mexico is 18, so anyone who is 18 can drink themselves into insensibility and feel the consequences the next day. Thus, the four adults will have to be vigilant and very good at watching the shore-sde alcohol consumption!
2007 Victory & Triumph
2008 Conquest & Liberty
2009 Valor, Freedom, and Splendor
2010 Glory and Liberty
I gotta go where it's warm.....
Over 40 victim of fate.....