The previous poster who mentioned drugs mules is absolutely right.
The lesson isn't about school rules or individual teachers etc, or about fair and not fair. It could be a lesson that saves his life one day about the reality of how people treat you when you break certain rules.
I think you should get your DS to do some supervised internet research into what happens to drugs mules.
And then write a list of your DS's excuses and reasons for wanting a lighter punishment at school... and see how they compare to the excuses of the patsy's who get life in prison in foreign gaols, or are indeed, executed.
I suspect the similiarities will be insightful.
I don't mean this as a punishment, but it's a way of learning that in some situations, it doesn't matter what your own intentions are, because you will be judged on your actions. Full stop, end of story, no small print or mitigating factors. Sometimes life really is that simple.
And yes, it's not fair, it should be shades of grey not black and white etc etc etc.
But in the society we live in, you never ever mess around with drugs, weapons or violence. Full stop. Because these are the solid lines, and if you cross those lines, you cannot expect anything different than the law.
Easily led doesn't cut it. Gullible doesn't cut it. And being otherwise a nice fine upstanding young man, nope that doesn't cut it either.
It's not a lesson in morality as such, it's a lesson in reality. Morality lets a tween / teen argue and debate and wriggle around. Morality let's their friends say that friendship is worth the risk. But reality is different, and probably the easier lesson to drum into him at this age and in this situation.
Better he learns it now than he gets used to whining about how unfair it is and then, he'll whine about how unfair the next thing is, and the next thing... until maybe he ends up on a beach in Thailand falling in love and of course he'll carry back that parcel... etc etc etc.
For what it's worth, I feel for him. But it's a good opportunity to set some clear guidelines in the murky waters of gaining and keeping friends.