So there are three open PCNs.
AG47328516, west of Thyra Grove, I think the signs are clear enough as you approach them head on.
AG47422547 & AG47266769 - the council will reject everything you say. An adjudicator at the tribunal might allow for inadequate signage, this being a personal opinion. Plenty of cases have been won and also lost. The council will probably throw in that you already knew about the restriction as you had already been sent a PCN, your earlier message implying that you paid two and didn't really look as to what you had done wrong and then did it again. That could be your undoing at the tribunal.
The second line argument is that you don't have the necessary degree of permanence in your lease in that the leasing company could demand the car back, if indeed the lease contains such a clause. The overall length of the lease is also material. That is a technical argument which would require us to see the entire lease and try to decide what an adjudicator would decide.
I'm a bit pessimistic about your situation (and also live in Barnet so know the most about locations and PCNs there) but that doesn't mean we can't beat the PCNs. You would have to risk £390 to fight all three PCNs to the end. It is your money and your choice.
If you don't live that far from the school perhaps the time has come to walk there which is the whole point of making streets near schools safer.
