"He is the tallest in the class."
Translation:C'est le plus grand de la classe.
As soon as you have an article in front of an adjective, that adjective gets the status of a noun : "le plus grand" = "le plus grand (garçon)".
Therefore, the rule about il/elle/on est + article + noun applies, with the replacement of il/elle/on by "c'est". The same applies to plural forms: ils/elles sont + article + noun = "ce sont + article + noun"