I stopped studying history at school in year 9 (age 14). Didn't stop me from learning about it, but I can't comment on what specifics were taught from then on. However, my impression from those who did carry on with the subject spent more time in history lessons learning how to be historians, rather than about history specifically.
Also, it seems to me that, in as much as anyone can ever learn about the horrors of life in the trenches of WWI from a book, does it matter if they learned about those at Passchendaele specifically? Or perhaps they learned about it under a different name? Passchendaele and the Third Battle of Ypres are one and the same; Flanders, which I expect sp *has* heard of, is also pretty much the same place. "In Flanders Fields" is one of the poems I *did* come across while at school, and while that dates from 1915 it was still written by a soldier serving at Ypres, and I don't think many children can have missed that one. Similarly Dulce et Decorum Est was part of the curriculum.
My expectation, anyway, is that these days the war is taught not as a list of locations, dates, and facts and figures, but as an experience, or as an attempt to describe what it was like to *live* at the time. I don't know how successful such attempts can ever be, but it's maybe not so bad an approach and I don't think it loses anything compared to teaching names and dates of particular battles.