I was a pupil at Isaac Newton School from 1959-1963.
I don't know what the official address was, as the school had two entrances.
One entrance was in Portobello Road, but what I think was the main entrance was in Lancaster Road.
I don't know if it was called Portobello Road School prior to Isaac Newton, but it was originally called Portobello Road School when built. If you look at the picture, there is a stone inset behind the lamp bearing the only visible word, School. If you do a street view on Google earth at another angle, you will see that the words 'Portobello Road' are indeed above the word school.
Incidentally, there was another school on the east side of Portobello Road called Colville School and the entrance is in Lonsdale Road. But in my day, there was an entrance in Portobello Road.