It didn't cause the slightest misunderstanding, in fact I'd be surprised if many people even noticed it, I certainly didn't until you pointed it out.
As a piece of written communication I would say it was entirely adequate. I would also argue that the school exists to prepare their pupils for their future life, especially their working life. I can't imagine a single instance in my long working life where such an innocuous mistake would make the slightest jot of difference to any outcome whatsoever.
I think this kind of pedantry is never useful, does nothing to aid communication and is more about showing off how clever you are than anything else.
I think being humble and gracious is a far more important and useful skill to nurture.
Mistakes happen, we're all human, as I said let he who is without sin cast the first stone - especially when there is absolutely no harm done.