Stefanidi attended the 1st High School of Pallini and won the National High School Championships. She broke the National High School record and the Championship record winning gold at the 2006 Gymnasiade. Growing up she broke all of the World age-group records for the ages of 11–14 and right after she turned 15, she broke the World Youth (under-18) record with a jump of 4.37 m. Her first international experience came at the age of 15, when she represented Greece at the 2005 World Youth Championships in Marrakesh, placing first with 4.30 m. At the 2007 World Youths, held in Ostrava, she was second with 4.25 m. In 2008, she won the bronze medal with 4.25 m at the World Junior Championships in Bydgoszcz, Poland. The same year she gained an athletic scholarship at Stanford University and began competing for the Stanford Cardinal. She received her master's degree in cognitive psychology with Dr. Gene A. Brewer at Arizona State University while training under the guidance of 2000 Olympic Champion Nick Hysong.