I've had exactly the same experience with reading a ruler on the
high school level. I wonder if they would be as weak if we used
metrics--maybe/maybe not...I think it's more than that; I think they
don't understand simple fractions such as 1/2", 1/4", etc. I've even
tried cutting out a piece of paper and telling the student, "Here's a
piece of gold; we will split it up. You cut it in 1/2, and I will
choose first." They can usually figure that out, after I pick the bigger
piece a few times.