It sounds like we've got a club of classically trained teachers working our way through the jazz syllabus! I got my Grade 1 jazz with distinction last term and am moving on to Grade 2 this term (gulp)
I'm working towards my DipABRSM on piano on the classical side, but like you felt that I should start at Grade 1 and work my way slowly through the grades. There's a different skill set that you need to acquire. Also the syllabus assumes knowledge of earlier scale patterns for things like the quick study and improvisations.
Try and find a soul mate who can help you along the way. I've been very fortunate to have a fellow teacher on the CT ABRSM course that I'm currently doing who has done Grade 5 jazz and has been very generous with ideas and guidance. Just sitting with someone and watching/listening to them improvise is really valuable.
Obviously you need to listen to lots of jazz!! Although the ABRSM Grade Jazz CDs are expensive I would say they are indispensable, particularly if you don't have jazz musicians around to help you out. Also get the books for the supporting tests. It's quite a big initial outlay, but one book does for all the grades so you get your money's worth.
When are you aiming to do the exam?
Karen