How a lesson begins has an enormous influence on what happens subsequently. Having clear and regular routines that are enforced without fail allow you a multitude of benefits, and I've picked out five for your reading pleasure.
You might be responsible for creating the only environment in which they are talked to in a respectful way, as a mature developing adult, on equal par to you in terms of dignity.
The boundaries you're uncomfortable enforcing are the very same ones that they NEED because the chances are, they aren't getting them elsewhere from the other people in their lives.
If you don't decide the values you stand for, you're never going to achieve coherence in anything, let alone behaviour.
A summary of the main learning points from the fantastic book, "Leading Better Behaviour" by Jarlath O'Brien.