cheapest way to get from yamaguchi to tokyo?

We are a group of 7 macedonian scouts who will be visiting the world scout jamboree in japan this year. After the jamboree we're planing a trip to Tokyo, so we're wondering what is the best and cheapest way to get there. We are a group of 7 people so i guess we're not eligible for any group tickets. I know that the answer is somewhere on the internet but we're working with a really tight schedule so some help from already experienced travelers .

Flying would likely be the fastest and cheapest. With discount air tickets you can get the flights down to around 10,000 yen per leg. When is the Jamboree?

http://www.japan-guide.com/e/e2364.html

overnight highway bus costs as little as &165;6500 one way depending on day of week
http://www.bushikaku.net/search/yamaguchi_tokyo/

sorry that should be 6500 yen

The bus is 13 hours long, and depending on when you travel it could cost more than a flight.