This is one of those topics where sources are important, not simply opinion.
From my superficial digging, the guidance varies, though it is worth pointing out that it is the AE site that gives some support for the comma - supporting the teacher's view. It is perhaps also worth noting that teaching of punctuation to ten-year-olds, like anything else, is likely to promote punctuation-by-rule before introducing flexibility. We need to take care not to apply post-graduate standards to ten-year-old punctuation
In my own defence
there is a world of a difference between:
When doubtful delegate.
- and -
When the robbers came I fought with one of them.
The first sentence demands a comma to be comprehensible - not least because delegate, without context, could be either a noun or a verb.
I suggest that the second reads easily and fluently without a comma.
And finally, Larry Trask states (see first link in previous post):
To begin with, forget anything you've ever been told about using a comma "wherever you would pause", or anything of the sort; this well-meaning advice is hopelessly misleading.
When reading aloud or singing, I always pause where a comma appears.
That does not mean that when writing I must put a comma wherever I would pause. I often pause where there is no comma in the text.
Commas are not meant to be breath-marks.