win 2k domain DNS

We have a win 2k domain with one server that is the PDC. we have dsl and about 25 PCs . What i need to do is to set the dhcp to give the DNS of the ineternet to some PCs and not the other PCs by my choice. since we are using proxy for the studensts and we would like to cut them off totaly for the net. but we still need to be on the same scope. (win 2k does not let me creat a second scope with the same mask even that i’m using differnet set of adreess but in the same range. please help
thank you