Explaination of Education systems in Iran
The most recent changes to the educational system took place in 2013 where primary education has been extended from 5 to 6 years, followed by 3 years of lower secondary and 3 years of upper secondary. In the new system, the 13th year, known as pre-university (pishdaneshgahi) has been absorbed into the upper secondary cycle and is no longer offered. This 6+3+3 year-system has become operational from September 2019 and thus the final one year known as pishdaneshgahi has been eliminated.
Students who complete the 12th year from the new system and pass final exams will receive their Certificate/Diploma of High School Completion degree. They will then be eligible to sit for the national entrance exam for universities of continue their education in any university abroad.
Office Of International And Scientific Cooperation has prepared a historic overview of Iran’s primary and secondary cycle for use as a quick reference when reviewing and evaluating academic transcripts of students from Iran.
Previous Educational Systems
Pre-1979 Islamic revolution [5+3+4]
Post-1979 Islamic Revolution (through 1995) [5+3+4]#
1996-2013 [5+3+3+1]

Current Educational System
2013 to present [6+3+3] (Practically implemented from September 2019)

Here’s a good link to a page on the UCAS website dedicated to Iran’s pre-university certificate and the recent changes made to the primary and secondary system: https://qips.ucas.com/qip/iran-pre-university-certificate-pishdaneshgahi