If there is a classless society which strrives to treat everyone the same, why do the pigs take the lead?

Unfortunately, the pigs behave the way humans often do, and that is part of the point of the story. There is theoretically no class, and theoretically everyone should be treated the same way. However, the pigs, who exemplify people who are more intelligent, believe they should lead and know what is best for everyone. The problem, of course, is that "Power corrupts, absolute power corrupts absolutely." Once Napoleon runs Snowball off and trains the puppies to be his "Secret Service" dogs, he creates classes that serve him and his wants.