"Sí, ellos siempre estudian para la clase."
Translation:Yes, they always study for class.
You don't need to add 'the class'. Sometimes in Spanish you add the 'el' or 'la' in front of a noun and you don't say 'the' in the sentence. So in this case, 'the class' or simply just 'class' would be acceptable.
I must have lost the semantics of "class". I don't see why there is an article missing ( in Spanish it stands straight.)