mobilization of ________ is a key to helping students who are the targets of bullying behavior.

The missing word is bystanders.

So the full sentence is:

Mobilization of bystanders is a key to helping students who are the targets of bullying behavior.

When classmates are encouraged, trained, and supported to act as upstanders instead of passive observers, they can intervene, comfort the victim, and report incidents, which significantly reduces bullying and its impact.