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.