My English study background

* The phrase that you had before ("17 to 21 with Australian teacher") was not a complete sentence. Also, the numbers "17 to 21" are confusing unless you explain that it means your age. 

** "Guy" is very casual (it would be like saying アースとラリア人のヤツ). So (unless he was a very bad teacher) it's better to say "man."

*** I added another comma to this sentence. The two commas separate the phrase "which was run by an Australian man." Notice that if you remove that phrase, the sentence still makes sense: "I went to the private conversation school once a week." The phrase about the Australian やつ***** adds more information, and helps us understand the question better. And the commas help set it apart. 

**** It's best to spell out small numbers. I usually recommend that if you can spell it with one or two words (例えば、"thirty-five" や "a hundred") then you should spell it. But if it requires three or more words (例えば "one hundred and one" や "three hundred and fourteen") then use numerals ("101" and "314"). So for formal writing, you would also spell out "seventeen" and "twenty one."

Another explanation of the rules for spelling numbers: Spell all whole number between one and ninety-nine and use numerals for larger numbers and fractions (例えば "33 1/3" や "1,420" や "3.14")

***** （笑）Just kidding.

My English study background