what kind of a novel is it?
Robinsonade – a genre of fiction
The Coral Island
A typical Robinsonade – a genre of fiction inspired by Daniel Defoe's "Robinson Crusoe" – and one of the most popular of its type, the book first went on sale in late 1857 and has never been out of print. 
what is 'the coral island'? A novel
wiki:10704693
The Coral Island [SEP] Introduction
It was the inspiration for William Golding's dystopian novel "Lord of the Flies" (1954), which inverted the morality of "The Coral Island"; in Ballantyne's story the children encounter evil, but in "Lord of the Flies" evil is within them. In the early 20th century, the novel was considered a classic for primary school children in the UK, and in the United States it was a staple of high-school suggested reading lists. Modern critics consider the book's worldview to be dated and imperialist, but although less popular today, "The Coral Island" was adapted into a four-part children's television drama broadcast by ITV in 2000.