That's because the Nazis had invaded the Soviet Union in the summer of 1941 and surrounded the city in an extended siege beginning that September. In following months, the city sought to establish supply lines from the Soviet interior and evacuate its citizens, often using a hazardous ice road across Lake Ladoga. It wasn't until January 1943 that a land corridor was created and the Red Army finally managed to drive off the Germans the following year. Altogether, the siege lasted nearly 900 days and resulted in the deaths of more than half of the city's population (somewhere between 400,000 and 1-1/2 million).
It wasn't until the fall of 1942 that the schools reopened and Korchnoi restarted the fifth grade, but he was not an especially good student. Even though the blockade had been broken it wasn't until the end of 1943 that things began to improve to the point that children were encouraged to enroll in the Pioneer's Palace, a nationwide government sponsored youth club where children talented in sports, music, art, chess, etc. could develop their talent.
Korchnoi, who had been taught chess by his father, attended with his friend where their instructor, a Candidate Master named Model, a former trainer of Botvinnik, didn't offer much in the way of instruction, but showed them studies and organized tournaments. At the time he was also interested in music and poetry, but those faded as he concentrated on chess. By the war's end he was studying under a master named Batuyev and later Vladimir Zak.
In 1947, Korchnoi won the Junior Championship of the USSR which was held in Leningrad and shared the title in 1948 when it was held in at Tallinn, Estonia.
In 1951, he earned the Soviet Master title, but not honestly. In 1951 more than 500,000 participants registered all over Russia for the preliminaries of the Chigorin memorial. In the final, held in Leningrad, Smyslov won first prize. Their individual game was drawn.
In the last round Korchnoi needed a win, but was paired against an experienced master and the game was adjourned in a dead drawn position. Because Korchnoi was a promising local player who had a number of supporters, including the organizers, they put pressure on his opponent by threatening not to pay him his prize money, unless he threw the game to Korchnoi when play resumed; he complied with the request by discovering a way to lose. Korchnoi admitted that he was aware of what was going on and in his youth laughed at his opponent's predicament, but when he looked back it was an incident he regretted.
It was because of this incident that the All Union Rating Committee, after checking the quality of games before awarding a higher title, rejected his bid for the Master title. His success did, however, allow him to be admitted to a semi-final for the next USSR Championship where managed to gain the Master title.
Leningrad 1951 Finals:
1) Smyslov 10.0-3.0
2-3) Taimanov and Aronin 8.5-4.5
4) Simagin 8.0-5.0
5-7) Kopylov, Korchnoi and Tolush 7.5-5.5
8) Estrin 6.0-7.0
9-10) Bivshev and Shamkovich 5.5-7.5
11-12) Klaman and Stolyar 5.0-8.0
13) Kamishov 4.0-9.0
14) Lutikov 2.5-10.5
Vladimir Alatotsev was also entered, but lost to Taimanov, Bivshev, Shamkovich and Lutikov and withdrew, so his games did not count.
I like the following game; it doesn't contain any flashy tactics but it's instructive. Aronin's little slip at move 13 doesn't look all that bad, but it allowed Korchnoi to obtain a small, but lasting advantage. And, when Aronin played it safe at move 22 it seems his fate was sealed.
Aronin (July 1920 - October 1982) was a meteorologist by profession and an IM. He played in eight USSR Championships which in those days were stronger than many international tournaments. His best finish was a tie for 2nd–4th places in 1950 at Moscow.
Very little is known about Aronin who never played outside the Soviet Union mostly because of Soviet chess politics. David Bronstein felt the Aronin, a player with a safe, positional style, should have been a Grandmaster. Based on limited data Chessmetrics puts his rating at 2674 in 1952.
Aronin was scheduled to play in the 1952 Interzonal Tournament in Stockholm, but was replaced by a high-ranking member of the USSR Chess Federation, Alexander Kotov.
Actually, it turned out to be a good move for the Soviets, if not for Aronin. Kotov went undefeated and finished first ahead of Taimanov and Petrosian (both of whom were also undefeated) by three full points.
It was apparent the five Soviet players had colluded to draw all of games against each other and they took the top five places. As a result, the FIDE decided to include the next three non-Soviet players in the Zurich Candidates Tournament: Gideon Stahlberg, Laszlo Szabo Svetozar Gligoric. Kotov bombed in the Candidates Tournament, finishing in 8th place.