I suspect that you may be limited to enforcing the provision of your divorce decree in family court. In other words, you may not be able to sue in regular civil court. You should contact a divorce lawyer in DE and inquire about enforcing this particular provision of your divorce decree. You and your attorney should look to the specific language of your divorce decree to be sure that the decree specified that your ex was to be responsible for half of "college" education expenses, and not just the vague term, "education," as stated in your post.
*Disclaimer: I am an attorney licensed to practice in Wisconsin, NOT Delaware. Any information provided is for informational purposes and does not create an attorney-client relationship. You should only rely on advise given to you by an attorney licensed to practice in Delaware.