It was just in the school_where_I once studied _that_we hold a party.
本句主要考查 it is ……that……这个强调句型。
where I once studied 是定语从句,修饰the school。
整句翻译:我们正是在我曾经就读过的学校举行的聚会。
补充:第一个空要用which 的话,应 为 in which I once studied。介词in不能省。
It was just in the school (where) I once studied (that) we hold a party.
that, where