I don't have any opinion on the Sussexes, but I have lived in and around Oxford for decades and, if the rumours are correct about their chosen school, I have a lot of sympathy with their issues with the commute and safe access. That part of town is notoriously awful for traffic and has a very high concentration of schools, making the school run beyond painful.
It is also in a rather tricky location with regard to exit routes - I would not want to have to remove a high-profile target from that area at speed in an emergency - there are very limited access routes and these are often heavily congested. If the decision to move was recent, it is possible that their security detail did not have the opportunity to assess the route/contingency plans during the school rush. The area is very much quieter when schools are closed - resembles the Hunger Games during term time. I can see how the reality of the situation may have dawned on everyone on day one.
They also have the option of lots of lovely, rural Prep schools nearer their home and which are likely to have much simpler access arrangements - some of the more remote ones will have multiple points of access to the grounds which could be used discreetly for anyone with a security concern - and could be used to evacuate at speed. I know it sounds ridiculous to most of us, but Harry has been subject to threats from hostile actors - they do have to consider, and plan for, the worst-case-scenario, no matter how indulgent it looks (I work in the security industry - access routes are a key vulnerability for any target and when you are committed to regular routine arrival/departure times and one point of exit, you increase the risk significantly).
I feel for them - it must feel awful to make such a big decision for your family and then have to change it so quickly.