Don't Judge a Book by Its Cover

Do not judge something by its appearance alone

Storm in a Teacup

A lot of commotion or fuss about a trivial matter

Ball Is in Your Court

It is your turn to take action or make a decision

Teacher's Pet

A student who is the teacher's favorite

Go Bananas

To become very excited, angry, or crazy

Out of the Blue

Unexpectedly, without warning