在She will be back from school.中,back是副词,句子意思很清楚.
在She will back from school.中,back是动词,句子意思根据上下文语境,有不同的理解,以下供你参考：
His garden backs onto a school.
他的花园背朝一所学校.
He backed his car from the school.
他把车倒出了学校.
She will be back from school.
她会得到校方的支持.