Lord of the Flies, by William Golding, is a novel about many things, including survival; by the time the boys are rescued from the island, many of them have done things for which they could be punished when they return to civilization. Though it is true the boys are all young, even the youngest of them understands the difference between right and wrong and knows that killing someone is not acceptable behavior. Nevertheless, there are several arguments to be made for not punishing the boys when they return to their "normal" lives.
First, none of the boys (with the possible exception of Roger) should be officially punished in a court of law. The oldest among them is thirteen, and most of the awful things they did were done collectively. Roger is the exception because he is the one who levered the boulder which crushed Piggy; other than that exception, the court is not the proper dispensary of justice for what happened on the island.
Second, no one in a position of authority can know with any certainty who did what while on the island and what the proper punishment for it should be. We already know that children see things from a different perspective than adults, so any retelling of the events is likely to be skewed.
Third, whatever any boy did while on the island will live with him for a very long time; in the case of the older boys, it is likely to last their entire lives. The guilt of participating in, watching, or failing to stop a murder is punishment enough for any of them.
Finally, the case can be made that the boys have learned their lessons about doing harm to others. If you choose, you can suggest that the boys not be punished but somehow monitored for the next ten years or so. If any of them again exhibit any savage behavior, they may then be dealt with as the court sees fit.
As either a closing or an opening argument, Golding's description of Ralph's thoughts at the end of the novel might serve you well:
Ralph wept for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy.
You can argue that enough has already been lost (innocence and friendship) and there should be no more.
The foundation of your stance rests on whether the boys should be held to the same legal standard on the island as they would be held to in "real" society. You can argue that because the boys were in a society that had only rudimentary guidelines, they shouldn't be held accountable from a legal standpoint. This is particularly true for the younger boys who participated who wouldn't be as knowledgable about social laws as the older boys.
Another angle to consider is the moral/ethical "laws" in play. From this side, several of the boys (Ralph, and Piggy before he was killed) are already being "punished."
So, when forming your opposition to the punishment of the boys, focus your argument on the stand that they did organize their own society with its own rules--of which murder was not considered. And, they were not a part of "real" society when the murder took place. You can also consider the idea of pre-meditation and intent, just as a court system would in society. Arguably, the boys were all caught up in mob mentality and were not fully aware that they were killing one of their own in the moment (which is in stark contrast to Piggy, later).
Finally, consider the psychology of adolescent development, which suggests that children don't gain a full understanding of the ramifications of their actions until around the age of 12; this is why an 8 year old cannot be given the death penalty for killing someone. The assumption is that children under the age of 12 are incapable of foreseeing the outcome and consequence of their actions (and, if they could, would steer away from actions with negative consequences). Some of the boys were over 12 and some were under, so you could argue that if a punishment were to occur, some should be punished less than others based on age/capacity for forsee consequences.