Emerson uses various rhetorical devices, such as the ones you have mentioned, making him sound much more conversational than cerebral and philosophical. He wants his ideas to be accessible to everyone—he isn't trying to write or speak to a learned audience only—since he's pushing for educational reform. He wants schools to do things differently than they have been doing, so he has to win people over. The effect of his use of many such devices is that he seems more congenial, more approachable, and even more reasonable and relatable, making his ideas about reform seem all the more friendly and reasonable too.
One example of Emerson's control of pacing comes early in the paragraph, as others have pointed out. The first two sentences, as you can see, are rather long and grammatically complete.
So to regard the young child, the young man, requires, no doubt, rare patience: a patience that nothing but faith in the medial forces of the soul can give. You see his sensualism; you see his want of those tastes and perceptions which make the power and safety of your character.
However, the third and fourth sentences are much shorter: "Very likely. But he has something else." In fact, the third sentence is not even a grammatically complete one. Emerson varies his syntax to retain his audience's interest as well as to maintain the conversational tone he adopts in order to make his ideas for reform more palatable. The shorter sentences mean more frequent full stops via the longer pauses we take when there is a period. More frequent stops makes him sound more deliberate as well as less formal in his delivery; thus, he is more likely to win his audience over.
He describes the patience and foresight required by the teacher, alluding to an "'eminent reformer' of whom it was said 'his patience could see in the bud of the aloe the blossom at the end of a hundred years.'" This allusion describes a person who was so patient, according to Emerson, that he could see a sprouting aloe plant and already foresee the flower that would take a hundred years to grow. The teacher must bring this level of patience and foresight to his classroom, especially as their class is full of children of different ages and abilities, and each requires such special care that, though the teacher hopes each morning for a "day of love and progress," they might end each day in "despair." Here, Emerson draws out his audience's sympathy for the teacher as well as helping them to realize how little can be accomplished in the best traditional classroom, let alone an average one.
Emerson employs an imperative sentence, or a command, when he tells his audience, "Try your design on the best school." He is attempting to communicate his belief that education needs to be reformed because even the best school is full of students that are too varied and different to learn well together, to make the teacher's job...
even remotely achievable. Emerson wants his audience to consider their ideas about education more closely, and so he literally makes them the subject of this sentence: the "you" is the implied subject, as in [You] Try your design." He wants each audience member to consider this command, and he relies on their good sense to come to the realization that traditional methods of education do not work.
Then, later in the paragraph, he employs a rhetorical question, one which he has structured in such a way as to elicit a uniform response that supports his own argument. He asks of the teacher, "Besides, how can he please himself with genius and foster modest virtue?" The implied answer is, of course, that he cannot. By compelling his audience to answer in the prescribed way, Emerson moves them closer to his way of thinking.
Having thus compelled both sympathy for the teacher as well as an understanding of why traditional education cannot work, Emerson ends the paragraph with an analogy: "the gentle teacher, who wished to be a Providence to youth, is grown a martinet, sore with suspicions; knows as much vice as the judge of a police court, and his love of learning is lost in the routine of grammars and books of elements." Rather than helping his students to learn and grow, the teacher becomes, instead, a disciplinarian only. He grows suspicious of his students and becomes, rather than a guiding light to his classes, more like the judge of a police court, accustomed to suspecting and accusing. How can students then receive a good education from him? They cannot.