In many novels, issues of class and society have to do with money, wealth, and poverty. In Mrs. Frisby and the Rats of NIMH, however, money and wealth aren't so much the issue as are questions of belonging and not belonging. The NIMH experiments have changed the rats so deeply that they are unable to fit in with their own kind any longer, but it is also unclear to them whom they can fit in with. They are outcasts from rat society. Since they can't just give up, the rats are forced to figure out how to build an entirely new society and to make up the rules that will govern it.
The NIMH experiments give the rats an amazing chance to completely reinvent their society.
These rats don't have a chance! Without clearly defined class and social rules, they will collapse into total rat chaos.