http://www.conspainmiami.org/Visados/STUDENT.htm
“Original official certificate of admission addressed to this Consulate in Spanish, issued in Spain, verifying enrollment as a full-time student in an official university or school in Spain, and verification of full payment of tuition (room and board). This letter must mention that you will be eligible for a credit towards your Bachelor’s Degree or other degree and indicate your course of study, duration of program, school’s address and the name and phone number of person to contact at school.
Information about the study program.”
I’ve read that different consulates have different requirements. Also, my idea about signing up for a course and not really taking it wouldn’t work as you have to prove your successful completion of a full-time course load to renew your student visa.