One of the difficulties for Christian parents raising their kids in the faith, is how to handle those years when their children are growing in independence, and are increasingly resistant to going to church. What is to be done in these circumstances?
What is required is for God to move in their hearts by his Spirit, to create deep faith. So pray, and pray some more.
Apart form this, I can only add a comment on what not to do.
I've observed a tactic employed by some parents in this situation that backfires more often than not. The tactic is to say to the adolescent something like: 'since you are a child under my care you have to keep going until you turn 16' (or until you finish year 11 or 12). This is problematic, because it does not deal with the heart of the youth nor their faith. What it actually does is provide them with hope - hope for soon being rid of church. Church then becomes a battle in perseverance in the child in all the wrong ways.
By setting an age limit before which attendance at church or youth group is compulsory, you've set a length of time to be endured and tolerated. Each Sunday until they hit 16, is another Sunday struck off the calendar. Even if they are attending the best church or youth group on the planet, their heart is not on the right place. Come the 16th birthday, they are out of there - and there is nothing you can do. It's part of your deal. Set a deadline, you set a horizon for escape and thus all is lost thereafter.
The trick then, is to do the harder job - namely to work with your teenager on their heart when the first signs of disaffection arise. Talking with them about their faith might be quite effective here. Talk over their doubts, their faith and how these things fit into the wider aspects of their life. You may end up with some excellent opportunities for 1 to 1 Bible reading and prayer. If this can be done, you are a world away from the conversation which ends up setting a deadline for attendance. Maybe this is un uphill battle - we are talking about teenagers - but at least try and try several times over. Who knows what God might do as a result of your prayers in these circumstances.