Benefits after age 18

You don't mention the type of benefits you are receiving - survivor benefits?

If survivor benefits, it seems unlikely you'll qualify once you've graduated from high school.

Here's an excerpt from this link :
http://www.ssa.gov/pubs/10085.html#benefits

Three months before your child's 18th birthday, we will send you a notice that benefits will end at age 18 unless your child is a full-time student at a secondary ( or elementary) school. If your child is younger than 19 and still attending a secondary or elementary school, he or she must notify us by completing a statement of attendance that has been certified by a school official. The benefits then will usually continue until he or she graduates, or until two months after reaching age 19, whichever comes first.