Lets look at some stereotypical assumptions as to why in general women don't earn as much as men.
Assumption 1.
Women are picked on more than men and seen more as possessions that should not get the high up jobs as the old boys network won't allow it. Now although this assumption can be true a lot a protections have been legally put in place to protect against this sort of direct negative discrimination.
Assumption 2.
Women at school have less incentive to try hard. This assumption again is incorrect and is as true for men as it is for women.

Here are some more positively constructive assumptions and it has nothing to do with right or left politics just some acute observations.

So what is it preventing more women from getting higher paid jobs.
Well in the 40's 50's 60's and 70's ect the way in which pupils were taught was more geared to men. Then as the National curriculum's came in the learning became more unisex but not really that well geared for either of the genders.

So how can you gear learning to men and to women in the same class.
Well lets first get it straight that boys and girls are not better or worse than each other at given topics it's simply how these topics are taught.
A girl for example learns by working on active methodology and building towards a concept.

So a girl thrives where there is a reasonable range of methods to learn and not to much immediate need to know in stone the concept in an environment where building the concept is assisted with good understanding of how a girl thinks and learns.
This is why you end up with many female cooks, nurses ect because they are active methodology based practical jobs where there is not to much demand on knowing the overall concept. However the fact that more women haven't gone into so much other areas is a demonstration of how far the education and work system still needs to come.

Boy's on the other hand need to know a concept of what they are doing and have a good method to follow with light well mannered discipline. This is why boys take on complex tasks more in technical fields because some of the boys have a good focus when they have been explained well the concept of what they are doing. How ever the fact that there is not so many male nurses ect is a demonstration of how far equality in education and the work place still needs to come.

Note.
Also some work environments still need to be adapted to allow less athletically strong people to get on with the work also doing this would help.