As a reading teacher, it is sometimes really tempting to skip these sorts of rules. They seem so insignificant, and they apply to such a small percentage of words.
But we we really want our students struggling over, say, the word insurance when they get older? No!
And so we teach it.
And, hey, all kids like the word sugar, right?
Sometimes, su says |shü|. And now our students are advanced enough to handle this fact.
Here's the card for your student's binder: