Just wanted to get your thoughts on this: would you expect your husband to pay school fees for your children / his step children?
If yes, would you expect his to continue to pay school fees if you separated?
My response would be yes to both.
Youre a family, and therefore everything should be shared equally. If you did separate, you should continue to pay the fees the same way you would for your biological children.
Edit: the same would apply if the genders were reversed.