Possible Clues:

• H.S. dropout's option
• High school degree equiv.
• Community coll. prerequisite, maybe
• High School deg. equivalent
• Substitute acquired by about half a million people a year: Abbr.
• H.S. dropout’s test
• Dropout's certification
• Many a H.S. dropout's goal ... and what's added to 17-, 25-, 36-, 51- and 60-Across
• Diploma alt.
• H.S. dropout's alternative