The teacher with her students ( )(plant) trees on the hill a

应当填 is planting,本句缺少谓语,因为at the moment所以用进行时.with,together with,along with 谓语单复数应当以所修饰的名词为准.
再问： at the moment不是也可以用过去式吗？
再答： 你说得有道理 例如： He is busy at the moment 他此刻很忙。 At the moment he was busy preparing his lessons . 那时，他正忙于准备功课 at that moment 强调在那时，that代指前后文提到的时间 所以本句填was planting 也可以。