Have each class learn one line of the song. Have that class stand up and sing their line. Tell the other kids not to listen to what the others are singing....kind of a reverse psychology. After singing line by line... ask if anyone could sing the whole song...all parts...- act very surprised that they all can do it. The kids will be very happy that they have "surprised" you by learning the whole song.
*Tip from my awesomely talented Mother-in-Law, Charlotte Stott Coleman who has been a Primary Chorister many times