You're right, carrots, it would actually be a good time for them to start doing that, once they're in the same school (DS1 doesn't do any after school activities) so could hopefully be on the same bus (of 2) as DS2, as well.
Elaine not to put you off at all! So, sorry in advance! My experience was rather different to yours, but:
We did the secondary school catchment move, a matter of 3 miles, renting a 3 storey town-house in the new catchment whilst looking for a house to buy but not wanting to be caught out by a collapsing chain etc. The new local primary had a place for DS1 who was half a term from finishing Y5 (you'll know they have to be in catchment in October, is it, in their Y6, hence the timing), but not for DS2 who was nearing the end of Y3 so had to stay and wait in his old school. Bear in mind the new primary was a 5 minute walk along a well-trafficked footpath to our renter, but this school finished at 3pm, whereas DS2, still at the old school, finished at 3.20pm. Note there's no parking anywhere near the new primary so there was no realistic 'grab and run' possible, and the boys were a bit discombobulated by the house move/school move etc. .
So I armed DS1 with a key, and alerted a friendly new neighbour that DS would be alone from 3.05 til 3.45-50pm when I'd return from picking DS2 up from 3 miles away. Fine.
Been doing it 3 weeks, no problems, when one afternoon I went to find the iron in the ground floor kitchen. I searched high and low, then DS1 said 'oh, it's up in my (2nd floor) bedroom'- um, why? Because he'd got in from school, then heard 'thumping' upstairs... so, after realising I was half an hour away, he gathered his courage, grabbed a weapon and went upstairs to investigate! Of course, he found nothing but I felt so bad! (Transpired it was the neighbours running up and down their adjacent stairs, through the party wall)- he was a bit happier when he realised what it was but we then instigated the 'grab and run' method where he'd be first out of school at 3, we'd run to the nearest place I could park my car, then drive swiftly to the other school where DS2 was sufficiently used to the 'new' arrangements' that he knew to wait by the school wall if we were late....
Luckily, 2 days before the school year ended, a place came up for DS2 to go into Y4 of the new school, problem solved.