Let's start now./ Let's begin our class.

我们开始上课吧。

Hello, boys and girls / children.

同学们好。

Good morning / afternoon, boys and girls / teacher / students / teachers, morning good / afternoon good.

早晨好/下午好，同学们/老师们。

Stand up / Sit down, please.

起立/请坐。

Who is on duty today？

今天谁值日？

这些句子涵盖了上课开始时的常见用语和礼仪，适合用于描述老师在教室上课的场景。