It blows my mind that you cannot go along in your own car and still be a chaperone.  yes, you do have plenty of time thank goodness and a school board meeting might be worth attending to raise some hay about this. 

Schools, by federal law,  have to accomodate all children without differential to special needs.  Sometimes you need to get loud about it.  I would attend a board meeting and drop the three little initials ’ADA’ on their lap.  While I hate to associated the D with CAH, it is more than that...it is making sure they accomodate your son’s special need.  You have given a viable solution and they should have to accomodate him with it.

Betsy