Spartan combat training was very rough and dangerous. What percentage of boys survived their training?
Our only source both reliable and substantial on Spartan life is Xenophon on the Constitution of the Lacedaimonians. Xenophon reports that the Spartan boys were required to supply themselves with a substantial portion of their own food ration, by foraging and stealing, and
[Lycurgus] made it a point of honour to steal as many cheeses as possible [from the altar of Artemis Orthia], but appointed others to scourge the thieves, meaning to show thereby that by enduring pain for a short time one may win lasting fame and felicity. It is shown herein that where there is need of swiftness, the slothful, as usual, gets little profit and many troubles.
and just prior to this he writes:
But why, if he believed stealing to be a fine thing, did he have the boy who was caught beaten with many stripes? I reply: Because in all cases men punish a learner for not carrying out properly whatever he is taught to do. So the Spartans chastise those who get caught for stealing badly.
and then further down again we read:
The penalty for shirking the duties was exclusion from all future honours. He thus caused not only the public authorities, but their relations also to take pains that the lads did not incur the contempt of their fellow citizens by flinching from their tasks.
Only for cowardice is the ultimate punishment, shunning, called on:
Often when sides are picked for a game of ball [the coward] is the odd man left out: in the chorus he is banished to the ignominious place; in the streets he is bound to make way; when he occupies a seat he must needs give it up, even to a junior; he must support his spinster relatives at home and must explain to them why they are old maids: he must make the best of a fireside without a wife, and yet pay forfeit for that: he may not stroll about with a cheerful countenance, nor behave as though he were a man of unsullied fame, or else he must submit to be beaten by his betters.
Small wonder, I think, that where such a load of dishonour is laid on the coward, death seems preferable to a life so dishonoured, so ignominious.
So our evidence from Xenophon is that the intent was to instill cunning, deceit, and resourcefulness, with pain, disgrace, and exclusion from future honours and privileges as the punishment for inadequacy in achievement.
While far from definitive, this suggests that, having not been abandoned at birth, the intent was not to weed out by attrition. On the contrary, it became the obligation of one's peers, relatives and superiors to assist each other in attaining one's duties. This philosophy of team-building is very similar to the modern boot camp's goal of turning out squads and platoons which leverage each other's strengths in order to make the whole stronger than the sum of its parts.
While not an exact percentage, according to Plutarch it does not seem to have been rare for them to die from dangerous activities performed during training, or from the punishments inflicted for failure during training:
The care with which the boys take over their stealing is illustrated by the story of the one who had stolen a fox cub and had it concealed inside his cloak: in order to escape detection he was prepared to have his insides clawed and bitten out by the animal, and even to die. This tale is certainly not incredible, judging from Spartan ephebes today. I have witnessed many of them dying under the lashes they received at the altar of Artemis Orthia.
(On Sparta Penguin Classic p. 23)