Airtag beeping sometimes. Why is that?

I have an airtag that I put it in my small child's school backpack. it is paired and set up to my iphone.

When my child came home from the bus today, my child's teacher aid asked me "what is this" while holding it in her hand. She said she found this in his backpack and that it was beeping in the classroom.

Now I didn't do anything. I didn't report is lost or did anything on" find my" app on my iphone at home.

I didn't get a chance to ask her how long did they hear the beep.

My question is, is this supposed to beep when it is away from me in his backpack at school? How long do they beep? Anyway to turn off the beeping?

Also once in a while, sometimes even at home where the bagpack is and where my iphone is, i could hear beep from the backpack. doesn't beep all the time at home but it does beep abour two or three times at home sometimes when we have the backpack at home with me.

thanks