12.6 percent of high-school students this year said they’d had a cigarette in the last month, down from 13.6 percent last year. quantify the change in relative terms

The change was 12.6 - 13.6, or -1.0, where the negative sign denotes a decrease.

Comparing this change to the original figure (13.6%), we get

-1.0
= -0.074, or -0.07.  This is the relative change.
13.6