I believe that each parent should be responsible for their own child[rens] education, whether they are in grammar, junior high, high school or college.
However, there are no hard and fast rules regarding this, either.
I know most divorced parents are required to pay for at least a portion of their child[rens] education, if they go to private schools and/or college.
As for the stepparent, I think it should be up to the individual stepparent as to whether or not he/she wants to contribute, money or otherwise, towards his/her step-kid[s] education.

Of course, there is more to education than just dollar signs. There is also checking homework, parent/teacher conferences and encouraging the kids to get the best grades that they can get. Naturally, the custodial parent is around the kids more often; therefore, he/she would be more responsible for checking to make sure homework is done. However, when the child[ren] see the non-custodial parent, that parent should take an interest in their grades and making sure their homework is done and any parent/teacher conferences that need attention.
If the stepparent wants to take part in checking homework, etc. as long as the bio-parent is OK with it and the child is OK with it, then there should not be too much of a problem. I had a situation where I checked one of my stepdaughter’s homework assignments and she got very upset and defensive, and my boyfriend, now husband, told me to back off. I checked it since no one else was checking and from what I can remember, the school was concerned about her completing her homework and they wanted her homework checked. At the time, I believe my stepdaughter was in junior high. Of course, after that I backed off and left the homework, etc. up to her bio-parents to be concerned about. Of course, this same stepdaughter had no problem with me paying for some of her college tuition at a later time. You gotta love it!
It really does depends on your relationship with your bio and/or step-kids as how much you can be or not be involved in their education.

Great point about the parents needing to continue their involvement–even after divorce. You are totally right about the step-kid situation. They can be much more defensive about the step-parent in my experience.