Thanks for using my Croatia forum. Regarding wild water rafting in Croatia there are four rivers where you can organize this type:
Dobra
Una
Cetina
Neretva
From my experience the best are Dobra and Cetina (near Omis) rivers. The travel agent who has more experience and the best reputation is Huck Finn d.o.o. Adventure Travel, Vukovarska 271 10 000 Zagreb Croatia. You can contact them via their web site to check if its possible to organize this trip for 25 young people. Their web site is : http://www.huck-finn.hr/rafting.htm their email address is info@huck-finn.hr.