The whole basis of these School Streets signs as applying only during school terms is flawed. The traffic orders generally apply throughout the year: if possible, check the one which applies to the specific site. As section 12.4.1 of
Chapter 7 of the Traffic Signs Manual explains:
12.4 Time of year
12.4.1. Where the restriction or prohibition applies for only part of the year, the appropriate dates are added to the sign. These can be expressed as specific dates, or months only, to accord with the traffic regulation order. Expressions such as “Term time” (to reflect school periods) are not permitted, but where a named day such as “Good Friday” is consistent throughout the country and familiar to road users, this may be used. References to bank or public holidays are permitted by the Regulations.
The issue is that, in essence, "term time" is not a well-defined legal term, so the lawyers aren't able to include it in the traffic order.
Local authorities pretend that the order applies only during term time. They may issue PCNs only in term time. But in so doing they are failing in their duty to implement the traffic order properly. As you make your bed, so must you lie. Taking account of irrelevant considerations (such as whether the school is in session) opens them to challenges of acting
ultra vires, which renders their actions void.
DfT advice about setting up school streets states
As School Streets usually operate only in term-time, flap-type signs are recommended that can be folded to show a blank face during school holidays. Arrangements will need to be made for carrying this out, including access and keyholding.
I believe that Cornwall does this. I don't know what their traffic orders say, but it is a recognised practice to have folding signs for restrictions which apply at irregular times, e.g. road closures around RAF Brize Norton when repatriating dead servicemen and women. Whenever the sign is displayed, it applies.
In my view, the DfT are remiss in failing to explain the legal impediments to using "term time only" on signs, why folding signs should be used instead and what the traffic order should say to allow the signs to be displayed only during term times. I shouldn't be surprised if Hackney, the local authority they quote as a pioneer, doesn't get it right, but for political reasons they don't want to draw attention to this.
For school streets, even with folding signs there's still the issue of the times that they apply. These are written in text which is so small that it's unreadable until you are so close that you're probably committed and would cause an accident if you stopped and tried to turn round. There is a Scottish version of the sign which has flashing lights when the restrictions are in operation. You don't need to know whether it's gone 09:15 or not. That seems a much better approach.
If you go down this line of argument, the problem which you will face is that the council will reject it. Adjudicators will find it a challenge: they have half an hour for each case and a knotty legal argument is the last thing they want. If they were to find in your favour, the local authority might not take it lying down but could seek a review, initially from a more senior adjudicator, but them by judicial review. Although this wouldn't cost you anything (the case is between the council and the tribunal), the very fact that such a case could arise makes adjudicators loth to allow them to arise.
Having said that, if you put in a frontal challenge to school streets and offer the adjudicator an off-ramp of some reason which applies to the particular circumstances, the adjudcator may allow your appeal to satisfy the requirements of justice without exposing the tribunal to the risks of judicial review.
In your case, the inadequacy of advance signage warning of the school streets restriction could be such an off-ramp. All that is provided is a small blue sign declaring "School Streets Ahead" without even saying where it is. A proper advance notice sign would show that a right turn has a restriction and what that restriction is.
