20 CFR § 404.745 - Evidence of school attendance for child age 18 or older.
If you apply for child's benefits as a student age 18 or over, we may ask for evidence you are attending school. We may also ask for evidence from the school you attend showing your status at the school. We will ask for the following evidence:
(b) If you apply before the school year has started and the school is not a high school, a letter of acceptance from the school, receipted bill, or other evidence showing you have enrolled or been accepted at that school.