Actually, Dhruva Mahārāja's mother, Sunīti, was his patha-pradarśaka-guru. Patha-pradarśaka-guru means "the guru, or the spiritual master, who shows the way." Such a guru is sometimes called śikṣā-guru. Although Nārada Muni was his dīkṣā-guru (initiating spiritual master), Sunīti, his mother, was the first who gave him instruction on how to achieve the favor of the Supreme Personality of Godhead. It is the duty of the śikṣā-guru or dīkṣā-guru to instruct the disciple in the right way, and it depends on the disciple to execute the process. According to śāstric injunctions, there is no difference between śikṣā-guru and dīkṣā-guru, and generally the śikṣā-guru later on becomes the dīkṣā-guru. Sunīti, however, being a woman, and specifically his mother, could not become Dhruva Mahārāja's dīkṣā-guru. Still, he was not less obliged to Sunīti. There was no question of carrying Nārada Muni to Vaikuṇṭhaloka, but Dhruva Mahārāja thought of his mother.
(Srimad Bhagavatam---4:12:32---purport).

It is stated in the Hari-bhakti-vilāsa that one should not accept initiation from a person who is not in the brahminical order if there is a fit person in the brahminical order present. This instruction is meant for those who are overly dependent on the mundane social order and is suitable for those who want to remain in mundane life. If one understands the truth of Kṛṣṇa consciousness and seriously desires to attain transcendental knowledge for the perfection of life, he can accept a spiritual master from any social status, provided the spiritual master is fully conversant with the science of Kṛṣṇa. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura also states that although one is situated as a brāhmaṇa, kṣatriya, vaiśya, śūdra, brahmacārī, vānaprastha, gṛhastha or sannyāsī, if he is conversant in the science of Kṛṣṇa he can become a spiritual master as vartma-pradarśaka-guru, dīkṣā-guru or śikṣā-guru. The spiritual master who first gives information about spiritual life is called the vartma-pradarśaka-guru, the spiritual master who initiates according to the regulations of the śāstras is called the dīkṣā-guru, and the spiritual master who gives instructions for elevation is called the śikṣā-guru. Factually the qualifications of a spiritual master depend on his knowledge of the science of Kṛṣṇa. It does not matter whether he is a brāhmaṇa, kṣatriya, sannyāsī or śūdra.
(Sri Caitanya Caritamrta---2:8:128---purport).

If one becomes a guru, he is automatically a brāhmaṇa. Sometimes a caste guru says that ye kṛṣṇa-tattva-vettā, sei guru haya means that one who is not a brāhmaṇa may become a śikṣā-guru or a vartma-pradarśaka-guru but not an initiator guru. According to such caste gurus, birth and family ties are considered foremost. However, the hereditary consideration is not acceptable to Vaiṣṇavas. The word guru is equally applicable to the vartma-pradarśaka-guru, śikṣā-guru and dīkṣā-guru. Unless we accept the principle enunciated by Śrī Caitanya Mahāprabhu, this Kṛṣṇa consciousness movement cannot spread all over the world. According to Śrī Caitanya Mahāprabhu’s intentions, pṛthivīte āche yata nagarādi-grāma sarvatra pracāra haibe mora nāma. Śrī Caitanya Mahāprabhu’s cult must be preached all over the world. This does not mean that people should take to His teachings and remain śūdras or caṇḍālas. As soon as one is trained as a pure Vaiṣṇava, he must be accepted as a bona fide brāhmaṇa. This is the essence of Śrī Caitanya Mahāprabhu’s instructions in this verse.
(Sri Caitanya Caritamrta---2:8:128---purport).

Everyone should consider his father to be his first teacher because by the mercy of one’s father one gets this body. The father is therefore the natural spiritual master. Our next spiritual master is he who initiates us into transcendental knowledge, and he is to be worshiped as much as I am. The spiritual master may be more than one. The spiritual master who instructs the disciple about spiritual matters is called the śikṣā-guru, and the spiritual master who initiates the disciple is called the dīkṣā-guru. Both of them are My representatives. There may be many spiritual masters who instruct, but the initiator spiritual master is one. A human being who takes advantage of these spiritual masters and, receiving proper knowledge from them, crosses the ocean of material existence is to be understood as having properly utilized his human form of life. He has practical knowledge that the ultimate interest of life, which is to be gained only in this human form, is to achieve spiritual perfection and thus be transferred back home, back to Godhead.
(Krsna Book).