don't lead untill he calms down because it will make him think that he has to be in lead and will therefor bolt even more!

Hi.

Try keeping him at the front of the ride and ask your friends not to overtake you. Try it in trot first then a slow collected canter. If he speeds up, ask your friends (before hand) to stop immediately. I'd try it in trot a few times and always trot and canter in different places so that he doesn't anticipate when you'll be speeding up, Also have his back and teeth checked to make sure there are no problems there and make sure your saddle fits properly. Horses will often run away from pain.

If all else fails, go back into the school and re-instill some manners through schooling, lots of transitions.

Hope that helps.