Here's where the prefects would greet the latecomers. And at any one time, there were many of these Nazis on guard gleefully waiting to book you. I outsmarted them by coming in from across the field, entering from Whitley Secondary School side. :)
There's another way behind the tennis court. There was a hole in the fence and you would have to brave through an deserted cemetery from the Swiss Cottage estate. I was not the only idiot doing that.