Quote Originally Posted by Tim the Enchanter
I don't think the fact that the Gryffs never took classes with any 'Claws was not due to any school policy, but just author error. Rowling is notoriously bad at maths, and there are some internal inconsistencies in the books, so Gryffindor-Ravenclaw classes probably do exist, they just don't appear in the books because the author forgot to include them.
I disagree with this... As long as there is another plausible explanation, I don't think that we can blame JKR like that...

At my school, we had a, b and c classes in each year. There was nothing really different about them, just a way to split the 100 students per year up. Since we didn't have mixed PE (so boys and girls had their own teachers) those classes were smaller, and we got mixed with one other class from the year. Some years it'd be a+b together in PE, and c alone, and then another time a+c, ...

I think it's just as likely that maybe in Ginny's year, there weren't a lot of students sorted into Gryffindor (I don't think we ever get to know anyone she shares her dormitory with, do we?), and they thought that having a class with four Gryffindors and another one with five Ravenclaws was just stupid, so they combined them.
And since in Harry's year there were enough Gryffindors to sensibly hold a class with, it wasn't necessary to have them share a class.

(Or maybe Ginny and Luna never shared that class, but Ginny told those boys off right before/after the class... or she had detention with McGonnagal and so she sat in the back of that class...)