Even if it is in church, the teacher must always give the youth the chance of receiving the Gospel.
What I mean by that, is that a lot of churches treat their youth members as automacially saved. Not because their parents are Christians or that they've been in the church long enough to be "Christians". But because perhaps it is a hard thing to inquire the hearts of the youth in regards to their salvation, for it could get "a little" intimate.
But isn't that the purpose of Christian fellowship? Intimate fellowship in love and in truth before our Lord.
Of course, this is not to say we treat all youth as unbelievers.
Offering a chance, an opportunity in our teaching, for even the oldest member youth or the most mature youth in Sunday school, to receiving the Gospel, yet not condescending their intellect, spiritual well-being.
That is definitely better than not offering this chance at all and assuming all of them are godly Christians.