Heart and Souls
All four of them are "tied" to a birth of a baby boy named Thomas in a car that was driving near the bus.  They must stay within a certain distance of him and cannot leave him.  As Thomas grows up, his parents and teachers are very worried that he is talking to "imaginary people," since only Thomas can see the four people.  Harrison, Milo, Penny, and Julia soon decide to disappear and not be a part of Thomas' life anymore.