Translation of message from Chinese to English
1
又体会到有同桌的喜悦感
I felt the joy of sitting at the same table again.
(maybe the boy she is crush on)

2
An old man probably sat by the door.
The girl asked him to keep (take care of) the book for her for a while.

The girl didn't know him before.
The stranger is the old man.

Two people. (I think 1 has nothing to do with 2.)

The old man left the message.

祝你學習進步
I wish you progress in your studies.

天天快樂
Happy every day.