they will probably ask you about eggs (tell them about all the different colors they could be!), and roosters crowing if they know what those are and not the one that lays the eggs! if you had a gentile roo to take that would be fun too! you may want to make a point that they can eat just about everything they can get their beaks on, but being careful to not let them eat too much or they get fat and won't lay eggs till they are thin again! oooh, and don't forget to mention that chickens go through molt at 18 months of age and may stop laying eggs but they will go back to it when they get their new feathers in! (too many people think their laying is done at 18mo and get rid of them)
depending on the special needs of these kids i would keep it simple. but remember that some of these kids are very smart and may be in that class due to behavioral issues or communication. if you can, have a few pics of other breeds of chickens with you so you can show them how varied they can be with attitudes, tameness, laying abilities (production breeds vs heritage!), and funky feathers like silkies, frizzles and polish!