I think I found the source of the poster: Плакат. Зина Портнова. Подвиг. Москва: 07. 12. 1984 г. Poster. Zina Portnova. Heroism. Moscow: 07. 12. 1984 y.

Плакат для советской школы. С описанием подвига советской патриотки Зинаиды Мартыновны Портновой. Poster for a soviet school. With a description of the heroism of the soviet patriot Zinaida Martynovna Portnova.

The text in the poster reads:
«Война застала ленинградскую пионерку Зину Портнову в деревне Зуя, куда она приехала на каникулы,— это неподалеку от станции Оболь Витебской области. В Оболи была создана подпольная комсомольско-молодежная организация «Юные мстители», и Зину избрали членом ее комитета. Она участвовала в дерзких операциях против врага, диверсиях, распространяла листовки, вела разведку по заданиям партизанского отряда имени К. Е. Ворошилова.

…Стоял декабрь 1943 года. Зина возвращалась с задания. В деревне Мостище ее выдал предатель. Фашисты схватили Зину, пытали, мучили. Ответом врагу было молчание Зины, ее презрение и ненависть, решимость бороться до конца. Во время одного из допросов, выбрав момент, Зина схватила со стола следователя пистолет и в упор выстрелила в гестаповца. Вбежавший на выстрел офицер был также убит наповал. Зина пыталась бежать, но фашисты настигли ее…

Отважная юная патриотка была зверски замучена, но до последней минуты оставалась стойкой, мужественной, несгибаемой. И Родина посмертно отметила ее подвиг высшим своим званием — званием Героя Советского Союза.

Сегодня многие пионерские отряды и дружины с честью носят имя Зины Портновой и делами подтверждают это право.»

Translated:
"The war found our Leningrad pioneer Zina Portnova while she was in the village of Zuya, where she came for vacation, not far from the Obol station in the Vitebsk region. An underground Komsomol-youth organization “Young Avengers” was created in Obol, and Zina was elected a member of its committee. She participated in daring operations against the enemy, sabotages, distributed leaflets, and conducted reconnaissance on assignments of the partisan detachment named after K. E. Voroshilov.

…It was December 1943. Zina was returning from a mission. In the village of Mostishche she was betrayed by a traitor. The Nazis captured Zina, tortured her, tormented her. The answer to the enemy was Zina’s silence, her contempt and hatred, her determination to fight to the end. During one of the interrogations, choosing the moment, Zina grabbed a pistol from the investigator’s table and shot point-blank at the Gestapo-man. The officer who ran in to hear the shot was also killed on the spot. Zina tried to escape, but the Nazis caught her…

The brave young patriot was brutally tortured, but until the last minute she remained steadfast, courageous, and unbending. And the Motherland posthumously celebrated her feat with its highest title - the title of Hero of the Soviet Union.

Today, many pioneer detachments and squads bear the name of Zina Portnova with honor and confirm this right with deeds."

Should be 13-14 regularily i think, depend on the date of birth, but those were still the years where the organisation of education was not entirely matured, and the education wasn’t strictly mandatory yet, so it might be that she just went to school year later.

I mean, her class probably ended prematurely due to war, so I guess she didn’t advance up higher in class…

Start of schooling was 8yo at the time. Methods of education and more specifically the start of schooling were a very contentious topic in the USSR. My mother once recalled to me that she published research that supported starting from 5 or 6 just because that was what the politickers supported.

Also found this while searching your question, which is an overview of education policy over time.