There are a few quick ways to gauge usage in English. The short answer is that bonfire is not all that common, but it's also used enough that most fluent speakers (especially in the US and UK) will be familiar with the term.
Oxford English Dictionary Frequency - The OED has a measure for frequency in current use for each of the words in its online dictionary. It is a scale out of 8, and its data is derived from a version of Google Ngrams ("Key to frequency"). The noun "bonfire" has a frequency of 5, which means the following:
Band 5 contains words which occur between 1 and 10 times per million words in typical modern English usage. These tend to be restricted to literate vocabulary associated with educated discourse, although such words may still be familiar within the context of that discourse. The shift away from the everyday language found in bands 8-6 is apparent in nouns (e.g. surveillance, assimilation, tumult, penchant, paraphrase, admixture), adjectives (e.g. conditional, cumulative, arithmetic, radioactive, symptomatic, authorized, Neolithic, discontinuous, preconceived, metrical), verbs (e.g. appropriate, comprehend, presuppose, perpetuate, encircle, jeopardize, subsist, gravitate, proscribe), and adverbs (e.g. markedly, empirically, functionally, disproportionately, ad hoc, exponentially, preferentially). This band also contains the most common adjectives derived from the names of philosophers and scientists (e.g. Aristotelian, Platonic, Cartesian, Newtonian, Darwinian, Marxist, Freudian). Most words which would be seen as distinctively educated, while not being abstruse, technical, or jargon, are found in this band.
About 4% of all non-obsolete OED entries are in Band 5.
Locally, I'd say bonfire is well-known enough that it'd go in this band, and largely in contexts like church retreats, scout troop outings, and outdoor parties.
Corpus searches in COCA and BNC - Corpuses like those at English Corpora can give valuable data about the frequency and use of a word. The Corpus of Contemporary American English and the British National Corpus feature hundreds of millions of words gathered from print media, interviews, broadcasts, and other formats. So I'll present the number of appearances in each corpus, the number of total entries in the corpus, the number of appearances per million words, and then provide some qualitative comments.
COCA: 1091 entries among 570,353,748 words, or 1.913 appearances per 1 million words.
BNC: 270 entries among 96,263,399 words, or 2.80 appearances per 1 million words.
This is about the rate I would expect for searching a commonly-known but not everyday word. For example, the food word cashew appears 277 times in COCA, and the more common peanut appears 5,057 times. Meanwhile, for bonfire, the hypernym fire is far more common with 83,757 entries in COCA. So bonfire certainly isn't an obscure word, but it's also situationally specific.
Google Ngram - Google uses the data it has from book scans to prepare data on how common words are in relation to one another. These Ngrams can be quite useful to see how common a word is. Here's one I prepared with bonfire:
The Ngram is good for showing related words with similar usage rates next to one another, and mapping trends in usage across time. For instance, we can see that bonfire, as a word, was once used more compared to the total corpus of words Google has. That might be worth investigating. Note, however, that it is easy to manipulate the Y axis by picking a word that's far more common:
Compared to fire, bonfire is a blip. So it's not like bonfire was once very common and now is less common.
In summary, bonfire is a commonly understood word that is only occasionally used. Its appearance in your English curriculum could be explained as a teacher's or a curriculum's interest in activities around bonfires, in adding some deeper vocabulary among more common words to spice up the reading and aid language acquisition, or something else.