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.)
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.)