How can we prevent bullying at school for teenagers ?

MORE THAN ONE WAY

You should ignore them.

Say thanks for not ignoring you.

Tell someone you trust.

Keep a record of what's happening.

Don't retaliate.

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."