在上课时不能玩东西的英文表达可以是：

"No playing during class."

这句话的来源可能是学校或老师规定的规则，意思是在上课期间不允许玩耍。

其他相关的英文表达还可以是：

"No distractions during class."
"Keep your focus on the lesson."
"Pay attention to the teacher."

以下是这些表达的中文翻译：

"No playing during class."：上课时不能玩东西。
"No distractions during class."：上课时不能分心。
"Keep your focus on the lesson."：将注意力集中在课堂上。
"Pay attention to the teacher."：注意听老师讲课。