选‘As’.（As）has been announced,the school will break up for holidays ahead of time.As常用于开头来引导原因状语从句.这里主句是一般将来时,从句是现在完成时.翻译：因为已经下达了通知,学校将提前取消假期.
其他选项可以变换句子结构,如下：
1.并列句：(That) has been announced,(and) the school will break up for holidays ahead of time.
2.it引导的主语从句：(It) has been announced (that) the school will break up for holidays ahead of time.
3.what引导的主语从句：(What) has been announced (is) that the school will break up for holidays ahead of time.