Rubbish at French!?

Hard to believe the Eiffel Tower was just meant to be a temporary exhibit in an expo.

I had a very interesting conversation with my ten year old son today about his French class. It was prompted my a comment his French teacher made during their lesson which he was quite upset about.
According to him she said, "You are rubbish at French!"
First of all I tried to get him to calm down using the techniques I've discussed with him before. Take a deep breath, relax every part of your body. Because if you relax the body, you relax the mind. Did he know that? No, he didn't.
Good. Now he does. Score one for dad.
Then I tried to understand the context. Did she mean just him, or the whole class?
The whole class.
Is she French?
Yes.
Is she old or young.
I don't know, but she is wrinkly.
Did she teach you last year.
No. I mean yes.
Would you say your class is rubbish at French?
No. I mean, not all.
Who in your class would you say is good at French?
Me, and about eight others.
And how many are in the class?
18
So half the class is quite good at French and the other half is rubbish.
Yes.
Okay, that sounds about average. And what was said to prompt this outburst from the French teacher?
We were all reading something.
Together?
Yes, and we came to a new word that we had just learned that day.
And who could read it out loud?
No-one.
Well, it doesn't sound a very nice thing for a teacher to say to a class.
No, it's not.
The next time she says something like that maybe you can just mutter under your breath, "Sacre bleu!"
What does that mean?
I'm not sure. It's what French people say when they're surprised.