I think any real web developer will tell you that W3Schools is bad news. W3Fools aims to correct this:
We feel ... that W3Schools is harming the community with inaccurate information. Like any other authoritative educational resource, W3Schools should both hold itself to, and be held to, the highest standards. We hope we can illuminate why W3Schools is a troublesome resource, why their faulty information is a detriment to the web, and what you (and they) can do about it.
The page goes on to describe myriad examples of W3Schools's inaccuracies.
Just like you should stop anyone using Internet Explorer 6, you should stop any budding web developers from using W3Schools.