I was doing the school run with both boys (3 and 6 - now 7) in a trailer up until this January; there is a short but sharp hill on the commute which was getting to be a major struggle. We upgraded Ben to a Frog 52 with gears in January and he sped up considerably; at that point I put him on his own bike for the school run.
I modified the route slightly at that point so we do around 1.5km on 20mph limit roads and 1.5km on shared paths.
I cycle behind him and in primary rather than secondary for most of the time.
My feeling before I started was that with the trailer, motorists usually behave pretty well, I was nervous that they wouldn't see both of us and pull in too soon after passing.
In practice we've been doing this 3 times a week for the last 9 months (inc. a similar journey to holiday clubs over the summer break) and I've found; the vast, vast majority of drivers have been very patient and my presence with a trailer has meant that they generally see us in plenty of time to slow down/manoeuvre appropriately. He has taken instruction very well and is generally more cautious than me, the first few months we did more or less continuous commentary on road positioning and observations of hazards - this does seem to be sticking and I now quiz him occasionally on whether he spotted a hazard and occasionally tell him to correct his road position.
We've also done a little bit of cycle touring together and this is where I'd advise caution; on national speed limit roads (and even 40 limits) drivers have a lot less patience for a cyclist only doing 7-8 mph; he can get his speed up to 10-15mph for brief periods and we do this where necessary.
Our biggest issue is crossing side roads on the shared path as we have to look in multiple directions at once...
He has started to get a little 'laid-back' about things now so I'm having to point out that he needs to watch what he's doing more (maybe once a week) - and, as with everything in life (and cycling in particular), it only takes one idiot in a ton or more of motor-vehicle or one lapse of concentration from Ben or I for a serious incident to occur...
Overall though, I think its the right decision; despite it being slightly more risky (for us) than driving him to school it should teach him that motor-vehicles don't have to be the default choice and may thus improve his health and life-span and it means he'll have had years of on-road experience before suddenly being allowed off on his own as a teenager...