That's not a contradiction. "Teach the correct rules of grammar at school" doesn't mean "teach people how to speak properly according to some prescriptive standard", it means "teach people (something approximating) the actual syntax of the English language so they can discuss it sensibly and understand something of what is actually happening when they put sentences together". (As opposed to the currently commonly taught "traditional grammar" which attempts to describe English but just gets it wrong.)

I agree they're descriptivist; by "the correct rules" I meant "the rules that govern the way people actually speak" (in which case "teaching the rules" would be more "showing people what the rules are" and not "instructing people which rules they should follow", the same way a biology class on digestion doesn't tell people how to digest.)

But then I haven't done an in-depth research on the education policy advocated on Language Log.