There are grounds to carry on, as far as I can see there are two possible points:
1) The traffic order (https://drive.google.com/uc?id=1iaURkPm5QHzt7H8qtWNH4E0jS7z2GxK3) creates a restriction "from a point 15.1 metres east of the party wall of Nos. 1 and 3 Portland Road", which would be towards the end of the school zig-zags:
(https://i.imgur.com/vZ8NWtn.png)
However the signs are actually far close to the property boundary than they should be, at less than 9 metres:
(https://i.imgur.com/Tl1FWjF.png)
General direction 2 of Schedule 8 to the TSRGD provides that:
2. An upright sign mentioned in paragraph 1 must be placed as near as practicable to the point the restriction or prohibition begins or ends.
It's not a sure thing, but this has won in a couple of prior cases, such as:
Stanmore Quality Surfacing Ltd v City of London (2230256434, 12 July 2023) (https://drive.google.com/file/d/12DH5F76jBQlKu2zwC0IAHZGs0oI5pRc4/view?usp=sharing)
Michael Dempsey v London Borough of Enfield (2230348292, 26 August 2023) (https://drive.google.com/uc?id=1gm5wYAaOUAoy98tzestBeUbQZq4M-dka)
The other argument is that by saying this:
(https://i.imgur.com/22dbg7k.png)
they have unlawfully fettered their discretion. They've been asked to consider the circumstances as a whole and they're telling us that they have refused to do so.
Lastly the grounds stated on the council website do not match the legislation, not by a million miles:
(https://i.imgur.com/g9AMAH6.png)
This has seen some recent success:
James Bettell v London Borough of Barnet (2180498755, 16 January 2019) (https://drive.google.com/uc?id=1Dob-9BvSfv--ox4_r64Oi-Y_LWK9QkZx), allowed
Tracey Allan v London Borough of Barnet (2180502295, 20 February 2019) (https://drive.google.com/uc?id=1WDh9frRbKwG0H7fGUTqqqTBiACRKlNia), refused
Sonya Karafistan v London Borough of Barnet (2230228494, 25 May 2023) (https://drive.google.com/uc?id=1h3CIvjuWnDlvZiLQRbASX5YgCL4w-Uuy), allowed
Martin Howley v London Borough of Ealing (2230274232, 29 June 2023) (https://drive.google.com/file/d/1qmCoQoX_3YPGLXui9U0k6XehnVhCHPVE/view?usp=sharing), allowed
None of these grounds are guaranteed to succeed, but you only need to win on one point. You therefore need to decide whether you want to carry on with the full amount in play, or pay now and forfeit the right to appeal.
If you want to carry on, I'm happy to represent you.
Send this as a letter in the post, make sure to get proof of postage:
Dear Royal Borough of Kingston upon Thames,
I make representations against penalty charge notices QT07890118, QT07860070 and QT07907902. I was unaware of the restriction and while I appreciate this is no excuse, in the totality of the circumstances I would ask that you consider excersiving your discretion not to enforce. As I will be abiding by the restriction in future, enforcement of these penalty charges would not serve any traffic management purpose.
Should you be unwilling to cancel all the penalty charges, I would ask that you at least cancel two of them and allow me to pay the third one at the 50% discounted rate.
Yours faithfully,