Its mission has been achieved.
The noun "School" is replaced by the possessive pronoun "its." It indicates that the mission is school's, not others.
\tt{NOTE:}NOTE: Do not mistake "its" and "it's." The two are not the same. "Its" is a possessive pronoun while "it's" is a shortcut for it is. Possessive pronouns are not written with apostrophes ('
(answer not mine)
