Is it true that Yahweh was part of a polytheistic religion?

It is true that there is evidence to suggest that the worship of Yahweh, the God of the Hebrews, was part of a polytheistic religion in ancient times. This idea is supported by archaeological and historical evidence, as well as by the biblical text itself.

Archaeological evidence from the ancient Near East suggests that the worship of Yahweh was originally one of many gods worshipped by the Israelites and other Canaanite peoples. For example, inscriptions and other artifacts from the Bronze and Iron Ages (ca. 3000-500 BCE) indicate that the Israelites were familiar with and may have even worshipped other gods such as Baal, El, and Asherah in addition to Yahweh.

Moreover, there are several passages in the Hebrew Bible that suggest that Yahweh was not always the sole God worshipped by the Israelites. For example, in Exodus 12:12, Yahweh declares that he will "execute judgment against all the gods of Egypt." This suggests that the ancient Israelites believed in the existence of other gods alongside Yahweh.

In addition, there are several passages in the Hebrew Bible that suggest that Yahweh was not always considered the supreme God in the ancient Israelite pantheon. For example, in Psalm 82:1, God is depicted as presiding over a council of "gods," suggesting that there were other divine beings in the Israelite religious worldview.

Finally, there is evidence to suggest that the Israelites did not fully embrace monotheism until relatively late in their history. The Hebrew Bible itself contains many passages in which the Israelites are accused of worshipping other gods, suggesting that the transition to monotheism was a gradual process.

In ancient times, to know the name of a god gave the worshipper access to its power. By calling a god's name, its presence could be assured. Yet the God of the Old Testament expressly forbade using his name in such a semi-magical way (Exodus 20:7). Neither is his name something to be discovered and manipulated by people; it is revealed by him to his people, to be revered. In Hebrew, the name of God is spelled:
יהוה In English, that transliterates as YHWH. In modern Hebrew, YHWH is pronounced "Yahweh." The English transliteration of YHWH is "Jehovah," which we get via Latin.

It's relevant to this question to know subtle differences in pronunciation of the divine name, to avoid inferring pagan ideas, or mixing it up with pagan deities. The name ‘Yahveh’ is NOT a pronunciation of YHWH. That's because according to the Talmud, Yahveh is the name of the Archangel Metatron, who because of his extreme Godliness, was given a new name by God that was almost the same as God's:
יהבה Those letters are transliterated as YHVH into English and the name is pronounced "Yahveh" in Hebrew.

The letter ו in Hebrew, the third letter in the name יהוה (YHWH) since Hebrew reads from right to left instead of from left to right, is the letter vav. Like many English letters, many Hebrew letters have more than one pronunciation. The letter vav is one of those letters and is pronounced either as a V or as a W, but in a few words, it is used to indicate the vowels O or U. Hebrew generally doesn't use vowels, but every once in a while it does, and when it does, it's some consonant that's being used as a vowel. In YHWH, the vav is always pronounced as a W: "YahWeh." It can't be construed as being pronounced as a V because of the name YHVH or "YahVeh" whose third letter is ב or bet. While the letter bet can be pronounced as either a V or a B in Hebrew, it is only ever pronounced as a B when there is a dot in the middle of it: בּ. Without that dot, it's always pronounced as a V. Since YHVH in the Talmud is stated as being very much like but still different than YHWH, the actual name of God, the vav in YHWH is always pronounced as a W and not a V or else the two names would be pronounced exactly the same and thus contradict the Talmud's teachings. These details show how easy it would be to misunderstand a name written in ancient Hebrew, thinking it was Yahweh, when it was actually that of (in this case) a created angel, Yahveh.

However, there's more to the matter of whether Yahweh was part of a polytheistic (hence pagan) religion, or not. The patriarchs are said to have worshipped a deity some think of as 'the God of the ancestors'. One Hebrew word for ancestor is rishon but only occurs once in the OT, in Leviticus 26:45, where it speaks of the covenant of the ancestors. The word aboth is quite common, meaning 'fathers, ancestors', and, as in Exodus 3:13 & 15 & 4:5 comes the phrase, "the God of your fathers" (or, 'ancestors'). Let me now quote from this scholarly book (when the author was Lecturer in Religious Studies at the University of Stirling, and had presented several religious programmes on television in Scotland.)

"The research of Albrecht Alt has shown that the worship of gods identified as 'the god of my father' (that is 'the god of the ancestors') was widespread among many tribes in the ancient world. We also know that the name 'El' was widely used as a name for local gods. In Canaan itself, the Ras Shamra texts depict El as the father of the gods and head of the pantheon at Ugarit..." Old Testament Faith, p. 38, Lion, 1986

Drane then says there are two ways of explaining this:

"Some (notably O. Eissfeldt and R. de Vaux) argue that worship of El and worship of Yahweh were originally quite distinct and separate... Professor F. M. Cross takes a completely different line, arguing that all these names (and others) used for God by the patriarchs referred to the one deity later called Yahweh. To speak of 'the god of the ancestors' was, in his view, just another way of referring to El, and 'Yahweh' was the way to address this one God in the context of worship. Cross's position is certainly closer to the theological stance of the Old Testament itself... (Ibid.)

Drane then goes on to say that the Israelites knew that Yahweh did much more than the Canaanite god El was ever supposed to have done. Yahweh was the God of the exodus, and had been the guiding force in their lives from the very beginning.

That what your history teacher told you sounded like blasphemy to you is unsurprising, given that Yahweh declares himself to his people to be the only true God; that all other claimants are false gods, no-gods, would-be-usurper-gods; that the idols of the nations surrounding Israel are lumps of metal, stone or wood, with no ability to see, to hear, or to do anything. The prophet Isaiah repeats that frequently, as do all the Old Testament prophets of the living God, Yahweh. So, my simple answer is, "No, Yahweh was never part of any polytheistic religion; he is utterly unique, the one and only Creator of life, light and eternity."