Can I file the 1098 if my 18 year old high school senior is taking college courses.

Yes, you should report the 1098-T on your return if you will be claiming her as a dependent.   And yes, she is considered to be pursuing a degree.   

Q.  Can I file the 1098-T if my 18 year old high school senior is taking college courses?

A. Yes. See explanation.

Q. With her being in high school, is she considered pursuing a degree?

A. No, or at least highly unlikely. See explanation.

The ability of the parent of a high school student to claim the education credit is limited.  If these courses are provided by an eligible college, this cost qualifies for the Lifetime Learning Credit (LLC). The school must have policy of granting college credit for that course, already taken, if the student ever enrolls there. In other words, it is a requirement that the course be a college credit course, even if the student isn't currently a college student. It’s not that the student is post-secondary, it’s that the course is post secondary. Books are not a qualifying expense, unless included in the course fee. You must have paid tuition to the college and not the high school in order to claim the expenses on your return.

https://www.taxact.com/support/22177/2023/education-credits-high-school-student-taking-college-classes?hideLayout=False

High school students are not usually eligible for the more generous American Opportunity Credit, unless they are officially enrolled as a degree candidate.