I don't think you can leave an island with a horse.
You go up to the front of the train and click on the conductor and he will ask you if you want to visit another island.
On every island, the Poptropica blimp is above Main Street. To leave Nabooti Island, return to Nabooti and walk right from the Museum.
you just get in the plane
any planet
See the engineer (he's in the very front of the train) and click on him and he'll ask if you want to get off the island.
To leave, use the Poptropica blimp on Main Street. (To complete the island, see the related question.)
Wait for principal to leave then you leave
Leave the castle, enter the city, and jump up into the blimp.
You have to win bingo and get the music CD. At the store,above the counter there is a music player. Put the CD in and put the volume up. That should make them leave
You click on the boat near where you came
just leave the fair and that's that.