Donnie with Down Syndrome has been taking lessons for a while, and it is time to teach him to read music on the lines and spaces.  First step:  how do you teach one to read music who does not know what a line is much less a space?  And a line is a THING.  But a space is a NOTHING!
How clever of me to notice there were lines on his shirt.  "Look, these are lines."  "No" Donnie corrected, "this is my work shirt."
So I drew lines on paper.  "This is a line....this is a line....(etc).  How many lines do we have?"  No comprehension.
If he does not understand line, there is no hope for space.
Then consider, a line note is not on the line, because you write on lines.  A line note is when the line goes through the note because if the note is on the line, it is a space note.  I had never seen note-reading as being so abstract.
Not knowing what to do, I applied the time honored technique of persistence and review.
This story has a happy ending.  Yesterday we again drew line notes and space notes.  Not perfectly, but Donnie understood the sequence of line/space/line/space pattern.  He could put the flash cards in correct order.  He knows patterns that go up and patterns that go down, and can translate that to the piano.  He can identify line notes and space notes.
All this has taken over a year, but Donnie is reading music written on lines and spaces.  WONDERFUL!