Giant's Bread by Agatha Christie

The friendship with Sebastian Levinne prospered and throve apace. Half the zest of it lay in the secrecy that had to be adopted. Vernon's mother would have been horrified if she had guessed at anything of the kind. The Levinnes would certainly not have been horrified -- but their gratification might have led to equally dire results.

School time passed on leaden wings for poor Joe, cooped up with a daily governess, who arrived every morning, and who subtly disapproved of her outspoken and rebellious pupil. Joe only lived for the holidays. As soon as they came, she and Vernon would set off to a secret meeting place where there was a convenient gap in a hedge. They had invented a code of whistles and many unnecessary signals. Sometimes Sebastian would be there before time -- lying on the bracken -- his yellow face and jutting out ears looking strangely at variance with his knickerbocker suit.