"Runaway, Come Home" from the Season Two episode, "City on the Edge of Forever (Flashbacks)".
Background
The boys and the other children are trapped on the school bus as it balances on a cliff, in the mountains. Ms. Crabtree went to get help but never came back.
The parents are worried about their missing children, thinking they ran away, get together to post missing persons fliers around town. Meanwhile, the children use the TV on the school bus and see the Channel 4 News report about themselves. Stan Marsh's father, Randy Marsh, along with the other parents, sing in harmony for the children to return home.
Lyrics
Runaway, come home. We love you just as you are.
Runaway, come home. We're sorry things went this far.