We did this with the junior highers and it seemed to keep them interested and excited:
First I passed out a blank envelope to each teen and told them to write their name and address on the front of the envelope (address it to themselves). We didn’t tell them why, so this piqued their curiosity.
Next, for the lesson part, I shared with them how I wanted to challenge them to minister to just one person this school year (this is good for a new school year, a new year, etc.). It didn’t have to be a classmate, but it could be anyone God had laid on their hearts that needed to be ministered to, preferably, one who was unsaved. I then shared several verses with them about evangelism and ministry, letting your light shine before men, etc.
Then, I passed out a notecard to each teen, and told them to write the name of the person they wanted to minister to on one side of the card. On the other side, they had to write a verse that would encourage or remind them of their person.
Then, they put the notecard inside the envelope, sealed it (I told them this is confidential) and gave their envelopes to us leaders. Once all had turned in their envelopes, we prayed over them.
I told the teens we would be sending these out to them sometime in the coming weeks or months in order to remind them of the challenge we made to them. They will not know when the envelopes will come, but we will probably wait about three to four months before we send them out to them. This will remind them to continue to minister in case they forget.