There are several good-bye expressions in Japanese, the choice among which depends on the degree of separation. “sayoonara” indicates that the speaker does not expect to see the person spoken to before she “turns a page in her life”; not until a new day arrives, or until fate brings the two together again. It sounds dramatic and ritualistic, and its daily use is largely restricted to school children taking leave of their teachers.
(between friends, expecting to see each other again fairly soon)
(taking leave from a professor’s office, for example)