First ask you guidance counselors for samples of letters.
Second our students have a form they fill out telling their interests, clubs and sports they have been part of their volunteer work.
Most letters of recommendation can contain skill/strengths, do they attend regularly, on time to class...prompt in handing in assignments, does homework, inter personal skills...personality..etc...
Hope this helps. I hate having to write letters for the weak students who suddenly decide art school is the "easy" option, which we know is not true.