Another option is to use the bus as a testing center. These might be engaging games at various workstations, and the games are what's being judged, not the students. The students are guest players comprising a focus group and giving feedback. Why is this worthy of school time? Because the games are didactic in nature, reinforce school subjects. That's why teacher feedback is especially valuable.
I've filed these ideas at the Math Forum, a file cabinet where I've stuffed a lot of my best ideas. I'm into using the Internet filing system to share useful ideas, appreciate others doing that.
The Math Forum versions have a somewhat different spin, as I bring in the Coffee Shops Network idea of having the games fund worthy causes of the player's choosing, a way of disbursing profits made from the game.
Players may record their patterns of disbursement, which becomes a portfolio, a chronicle of their heroics. Of such stuff one's identity and reputation is comprised, so these games may be said to build character.