Even though his crimes were never more serious than putting guinea-pigs into the master's desk, yet that sort of conduct time after time is not to be tolerated in any school.

The example set by a mischievous boy to his fellows is not good; and if his scrapes are winked at always, the time will come when others will be encouraged to follow in his steps, and behave badly too.
Sam, no doubt, deserved the punishment he got; and because one bad boy who is punished is no worse than a dozen bad boys who get off, that does not make him out a good boy, or a boy more hardly treated than he merited.
Scapegraces are boys who, being mischievously inclined, are constantly transgressing the line between right and wrong.

Up to a certain point, a boy of good spirits and fond of his joke, is as jolly a boy as one could desire; but when his good spirits break the bounds of order, and his jokes interfere with necessary authority, then it is time for him to be reminded nothing ought to be carried too far in this world.
One last word about scapegraces.

Don't, like Sam, get it into your heads that you are destined to get into scrapes, and that therefore it is no use trying to keep out of them.

That would be a proof of nothing but your silliness.