Students' Most Hated Phrase

Today I was reviewing for our final exam when a student asked me when to use a certain grammar structure, one that we have been using for 3/4 of the year.  One that I have reviewed over. and over. and over again.  Most recently yesterday.  And he asked as if this were a legitimate question that needed clarifying.  Three days before our exam.

So I was concerned.  When teachers field questions that to them seem obvious, or even needless, it is difficult not to show a level of such concern or annoyance.  To be honest, I was very annoyed.  This is a structure that students see the level before mine and, added to it, the majority of our level.  So I answer the question and say,

"You should know this."

Because he should!

Flashback to yesterday after school as I'm talkin shop with another teacher.  He remarks how in the final exam review period it can be frustrating to see how many students ask thoughtless questions that have been discussed all year long.  We commiserate and feel much better knowing we are not the only ones to have an intellectually delinquent set of children.  And then he remarks that he has actually read that a student's most hated phrase by teachers is, in fact, you guessed it: You should know this.

Why? Because they should know this! It! Whatever 'it' is! However, that doesn't change how students - all students, apparently - view this statement.

What we say: You should know this.
What they hear: You are stupid.

And today when those words came so effortlessly out of my own mouth, like four little innocent messengers of truth and justice, how I didn't know that they would be received as four insufferable harbingers of mockery.

So I stopped the lesson and asked the kids what they thought about that phrase.   And being high schoolers - extremely sophisticated and scholarly - they thought about it and said the exact same thing.

"Makes us feel stupid."

Eloquently put.

We then discussed various ways to achieve the actual meaning (I am concerned that you do not know this information after using it so often and for so long) without the added implication (...you idiot.) We agreed that the best way to do this is to explain the thinking behind those four words instead of using them, even if it takes longer.  That way the students will understand you but not feel berated.

What about you? Can you think of something you hated more than this? Is there nothing wrong with this phrase?