I too was at school in Scotland in the late 50's and early 60's

I think the last verses are

What came over Biddy oh they cannot say
But these are the tales that they'll tell you today
How Biddy and washing just vanished away
on a very fine morning in glorious May

She was washing her clothes in the river that day
and tramping her blankets as if it were play
When down came a spate and washed Biddy they say,
Right out to the ocean and far,far away.