sheep that go astray

I'm working my way through 'The Godbearing Life,' by Kendra Dean and Ron Foster.  The book takes a fresh look at the important role that adults have in being 'Godbearers' to students as oppose to program leaders.

As the authors begin the 3rd chapter, they refer to Jesus' parable about the lost sheep and how every student that enters into our 'flock' is now under our care.  The authors continue on and say that we will be held accountable for every sheep that is lost and never returns.

While I am familiar with this parable, this comparision is frightening to me.  There is a large amount of students who have come through our doors in years past who have since stopped attending and/or lost interest attending our church... Many of them, to my knowledge, did not leave to attend a different church.  While I realize being a minister calls us to love and care for the sheep of our flock, it also seems like an unreal expectation to be personally responsible for every single one.

With many of the teens, though not all (regrettably), there have been follow up calls/messages inviting them back to the church.  However, at the end of the day, they are free to do as they please.

How responsible is the minister?  How responsible is the local church for sheep that have gone astray?  How do you interpret this in light of the parable that Jesus spoke?