Aristotle (384-322 B.C.) was born in Macedonia. At age 18 he went to Athens and joined Plato's Academy, where he remained for twenty years; his works are full of echoes of Plato. Later he founded his own school, the Lyceum. Socrates never had a school; he philosophized informally, in conversation. Plato's academy was at first probably something like a club, but it developed into a large educational institution like a university. Aristotle's school, the Lyceum, had hundreds or (according to some accounts) thousands of students. Philosophical education became a large industry in Athens, attracting students from all over the Mediterranean.
We imagine that in the Academy Plato gave lectures or talks or seminars or held conversations, but we have no record of what he said. He did not write it down. In fact, as he explains in Letter VII 341c, he did not think that philosophy could be learned from writings, but only in conversation: "after much converse about the matter itself and a life lived together, suddenly a light, as it were, is kindled in one soul by a flame that leaps to it from another". Aristotle in some of his writings reports and criticizes theories and arguments he ascribes to Plato that are not found in Plato's dialogues, and it is assumed that these were things Aristotle heard Plato say while he was a member of the Academy. In one place Aristotle uses the word "we" to refer to himself and the other members of Plato's school, though even there he is rejecting Plato's key doctrine: "Further, of the ways in which we prove that the Forms exist, none is convincing..." (Metaphysics, I.9).
We suppose that in the Lyceum Aristotle gave lectures, and the surviving works of Aristotle seem to be based in some way on such lectures. They do not seem to be works addressed to the general public: they are very concise, often cryptic. Presumably Aristotle elaborated on them in some way when they were used in his school. Perhaps he agreed with...