Fired
In season 2, the actresses Whelchel, Cohn, and Fields were surprised to see that they were the only members of the cast remaining; They had no idea that everyone else had been fired between seasons 1 and 2.
Though Molly Ringwald became incredibly famous soon after her departure from The Facts of Life, some of the girls who were let go relatively early on did appear in other seasons. It seems as if The Facts of Life producers actually did her a favor by letting her go when they did; otherwise, her career could have looked very different.