How do you deal with someone you care about disrespecting you?

I mentor a group of high school kids and up until recently, I thought they had some level of respect for me. During two separate sessions, two of the girls told lies to the entire group in my presence. Now, am I right to feel that they don't respect me because they lied about me to others and had the nerve to do it while I was sitting right there? Do you think I am over reacting? The reason they both lied is because they were given a task that they did not complete and when called out (by another group member - not me - they both felt ashamed they had not finished the task and sought someone else to blame). Is this common behavior in teens and I just happened to miss that in all of my reading?