Sometimes THE PROBLEM is NOT THE KIDS, BUT THE LATE ARRIVING TEACHERS !!  

15 minutes is not a very early arrival. It will go by fast, and I don't know a teacher who couldn't use a little extra help in that 15 minutes. Involving the kids in the room preparation can be great personal time with students, and make them feel INVESTED in the class.

Pre-Workshop Activities:

Drama:  Have early arrivers help you sort or create props and staging.

Cooking: Have early arrivers get out cooking supplies and perhaps have them prepare a little pre-class snack, like trail mix.

Computers: Lots of the software you have already used have games in them which you can assign for the kids to go back and play again (which they will gladly do).

Art:  Have the early arriving students make a BIBLE VERSE SIGN for the DAY taken from the lesson.

Video:  Ask them to cue up the video and test the sound volume, and pop the popcorn.

Games:  Arrange and Test the game equipment. Help lay out the game supplies.

For regular early arrivers, turn them into "GO-FERs" ...making copies, gathering supplies, posting signs, etc.

Also invite an early-arriver to FIND THE SCRIPTURE and prepare to read part of it for the class.