The cartoon in question concerns the expansion of American imperialism at the end of the nineteenth century. We see Uncle Sam (representing the United States) in the role of the schoolmaster. At the front of the classroom sit students representing Hawaii, Puerto Rico, Cuba, and the Philippines. These territories had just come under American control. These "students" are depicted as unwilling and somewhat defiant pupils. They clearly are not happy to be under the tutelage of their new teacher. They are also drawn in such a way that emphasizes racial stereotypes.
Elsewhere in the classroom, we see other nonwhite members of American society at the time. A Black child is cleaning the windows. A Native American child is off to the side, reading a book upside down. An Asian child stands outside the door. These figures are also depicted in a racially derogatory manner. By setting them apart from the rest of the class, the cartoonist is suggesting that these minorities are not good students of American civilization.
In the back of the classroom are the "good" students. They represent territories that were already absorbed into the United States, such as Texas, Alaska, and California. We can see by their fine clothing and compliant behavior that they have taken Uncle Sam's lessons to heart.
On the blackboard in the back of the room is a message defending imperialism as a mechanism of civilization. Essentially, it says that people often do not agree to be governed by imperial powers but that it is for their own good, since someday, the colonies will have learned enough from their imperial masters to govern themselves. Indeed, the caption has Uncle Sam saying that if these new students listen to him, they will soon be as glad to be there as the well-behaved children behind them.
As the United States ramped up its imperial endeavors toward the end of the nineteenth century, there were many who argued that subjecting other peoples to American rule without their consent was contradictory to America's founding principles of self-rule. Had not the American Revolution been fought to rid itself of rule by an imperial power? This cartoon seems to be in answer to that question. It is providing the patronizing counterargument that the "civilizing" nature of the United States would be a beneficial thing. These imperialists argued that the United States would elevate its colonized people to a position in which they would someday be able to govern themselves.