Teachers more likely to “mark up” girls' work

As a teacher, I guess I'd agree. But I think that the bit about girls tending to have better attitudes toward school is the key point. It's much easier to be err on the side of giving marks (not awarding marks for incorrectness, but I guess trying to find the reasoning in partially correct answers?), providing more detailed feedback etc when you know the student will appreciate it or displays what the article refers to as a "school friendly" attitude.

"Mr Schleicher says the study shows there is nothing innate, immutable or inevitable about gender differences in education."

natural selection provides for gender (and, it must be said, racial) differences. we waste a lot of resources pretending the world works like we wish it did rather than how it actually does.