When a person is highly educated and acts strictly on moral principles, he is called highly learned. A person conversant in different departments of knowledge is called educated, and because he acts on moral principles, he is called morally stout. Together, these two factors constitute learning.
Kṛṣṇa's receiving education from Sāndīpani Muni is described by Śrī Nārada Muni as follows: "In the beginning, Lord Brahmā and others are like clouds of evaporated water from the great ocean of Kṛṣṇa. In other words, Brahmā first received the Vedic education from Kṛṣṇa, as the clouds receive water from the ocean. That Vedic education or instruction which was spoken by Brahmā to the world was then reposed upon the mountain of Sāndīpani Muni. Sāndīpani Muni's instructions to Kṛṣṇa are like a reservoir of water on the mountain, which flows as a river and goes again to mix with the source, the ocean of Kṛṣṇa." To be more clear, the idea is that Kṛṣṇa actually cannot be instructed by anyone, just as the ocean does not receive water from any source but itself. It only appears that the rivers are pouring water into the ocean. So it is clear that Brahmā received his education from Kṛṣṇa, and from Brahmā, via the disciplic succession, this Vedic instruction was distributed. Sāndīpani Muni is likened to the river which is flowing down again to that same original ocean of Kṛṣṇa.