In this use, "feed on it" means that it will increase the students' agitation.
Student or Learner
There will be times when you have to raise your voice in the classroom. ...In fact, it will make matters worse. They will see that you have lost control and they will feed on it.
Does "feed on" mean "make use of something" or "become stronger"? By hearing the teacher's shout, how do students feed on it?
They will take advantage of your loss of control and disrupt the lesson.