Quite a decent translation, to my thinking. I admire how much literal accuracy you've retained, despite the challenges of the poetic form.
"At life's beginning" may be a bit too literal though. It is hard to think of anyone speaking that way in English - about school "at life's beginning", instead of, say, "in my early years" "in my childhood" "when I was young", "early in life" etc.
A woman very meek, in shabby dress,
who had a nonetheless majestic poise,
was there to strictly guide our early steps.
Consider perhaps "rather" (or "somewhat") to avoid putting undue emphasis on "meek", and some adjustments to avoid the split infinitive - perhaps something like this:
"A woman rather meek, in shabby dress,
Who nonetheless possessed a stately poise,
With strictness supervised our early steps."