The Heat of Day by March Hastings

A summer camp for rich girls

'Away from home at a dangerous age, it became their summer of awakening.'

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From the back:

It was an exclusive camp for well-bred young ladies... teenage girls, on the threshold of womanhood... long-legged and sweetly-sensitive, eager for an end of innocence, restless with the nagging needs of unfulfilled appetites... anxious to explore the secret avenues of pleasure spoken of in hushed whispers... willing to follow the strange paths that led to forbidden ecstasies... the only kind of ecstasy available to them in this world without men.