The effects of relevant factors related to self-assessment have not been systematically investigated. We explored four factors and their effects on self-assessment and self-efficacy: (1) feedback (with vs without), (2) subject matter (Spanish vs mathematics), (3) year level (K7 vs K10 vs K11), and (4) gender. The participants included 64 secondary education students who self-assessed during a set of Spanish and mathematics activities while being video-recorded. Data came from think-aloud protocols, direct observations, and self-reported instruments. The use of self-assessment strategies and criteria was more frequent and advanced without feedback and in females. There were differences in the self-assessment of Spanish and mathematics. As for year level, results showed more similarities than expected, though the use of advanced strategies and criteria varied across levels. Additionally, none of the factors had significant effects on self-efficacy. This study opens a new avenue for self-assessment research, unveiling the black box of self-assessment.