Reading levels for older students can be determined by Independent Reading Inventories that check up at least through 12th grade. There are many very good ones out there.
The paragraph below is from an article on the site:
"If you don't have an IRI, you could use any leveled book, do a running record and have the child retell to determine the following: Independent Level: Students can read these "good fit" books alone with a high level of success. Instructional Level: This level of text would have a few challenges, which focused guided instruction will overcome. Frustration Level: This material is too difficult in either content, vocabulary or both. A steady diet of frustration level texts will produce unenthusiastic and nonfluent readers. Comparing instructional levels from one round of assessments to the next is a quick and easy way to measure individual growth."
There are charts available that compare the reading levels using many different leveling systems: Lexile, F & P levels, Intervention levels, DRA Levels, etc. If you google "Comparing Reading Levels", you'll see several options.
A great site for finding the levels is on the Schloastic.com site, http://www.scholastic.com/teachers/
Go to "teachers" then Book Wizard. It will give reading levels up to 12.8. You can request books by Grade Level Equivalent, Guided Reading (F & P), DRA and Lexile levels.