后天我们要开学了，用英语怎说？

Our school will start at the day after tomorrow
or
We will start to our school in two days

the day after tomorrow 后面 是时间状语

放句首和句尾都可以 中间写

We will begin to study new knowledge

We will start the classes the day after tomorrow.

威尼英语标准翻译

We'll be back school the day after tomorrow

Our school is going to start the day after tomorrow.

Our new semester will begin the day after tomorrow
如果用semester 会让老师另眼相看的。term 比较小学化常见些。
我这里引用bbghgf 的翻译，比较完美。