As @OHMomof2 notes, schools don’t expect the impossible or even the difficult – 15 hours is not driving distance, and schools would not expect you to make that trip just to show interest.
Other ways to demonstrate interest are signing up for the mailing list, following the school on social media (making sure that the student account used to follow the school is “clean” with no scandalous images etc.) If the school comes to a college fair in the area, go, meet the rep, sign the sheet. Request an alumni interview. Once you apply, check the application portal, open the emails, click the links.