I've mentioned a few parents that are convinced their students are "dumb." Their words not mine. When you have people that are supposed to believe in you the most telling you that achieving your goals is impossible, of course your grades and self confidence will reflect that more times than not. What do you do? How do you look at a kid and say "your parents are wrong, listen to me?" Well, depending on your personality and commitment, pretty darn easy.
We've all had students that are working their tails off and getting "nowhere." They don't see a change, the numbers aren't changing, and it isn't getting easier. As a teacher, you can tell the kids they are more than numbers. Help them see that even though they can't see immediate results, they're getting experience and becoming more comfortable finding the answers they are seeking. You can show them that identifying and overcoming weaknesses is part of growing up.
Again, kids need to know that failure doesn't mean they are worthless, stupid, hopeless, or anything other than that they have not mastered a particular subject. It can be overcome, and it will. Success is not only possible, it's probable if we change the way we talk with our kids.
Normally not so cheery over here, but something about parents not believing in their kids gets me fired up.
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