Actually the story of Good Morning Miss Bliss is even more interesting. The original pilot episode actually was made and aired in 1987 on NBC, and was even more different than the rest of the series. None of the student characters were in the pilot that were in the main series, and others were played by Brian Austin Green (of Beverly Hills 90210 and Megan Fox fame), Jaleel White (Family Matters), and Jonathon Brandis (seaQuest DSV). Haley Mills was Miss Bliss, but other characters like Mr. Belding were played by different actors.
I guess the pilot didn't do well so NBC passed, Disney picked up the rights and retooled the show and debuted it in late 1988.