5 sentences

1. 上课时千万不要分心。

   shàng​kè shí qiān​wàn bù yào fēn​xīn.

   Make sure not to get distracted in class.

   Source: Dong Chinese Dictionary

2. 上课时间到了。

   shàng​kè shí​jiān dào le.

   It's time for class.

   Source: OpenSubtitles

3. 上课时，有些学生总爱搞小动作。

   shàng​kè shí, yǒu​xiē xué​sheng zǒng ài gǎo xiǎo​dòng​zuò.

   Some students always fidget or engage in sneaky activities during class.

   Source: Dong Chinese Dictionary

4. 他今天听课时心不在焉，总是在看窗外。

   tā jīn​tiān tīng​kè shí xīn​bù​zài​yān, zǒng​shì zài kàn chuāng wài.

   He was absent-minded during the lecture today, always looking out the window.

   Source: Dong Chinese Dictionary

5. 上课时她是我们的老师，下课以后是我们的朋友，而且是很好的朋友。

   shàng​kè shí tā shì wǒ​men de lǎo​shī, xià​kè yǐ​hòu shì wǒ​men de péng​you, ér​qiě shì hěn hǎo de péng​you.

   In class she's our teacher; after class she's our friend, and a good friend too.

   Source: Tatoeba