Been trying to find this one for ages although I think it's somewhere in one of my compilations. Aoi Sankaku Jougi (青い三角定規...Blue Triangle [as in the one in your school pencil case]）was a short-lived folk group that managed to sing one of the evergreen songs of the 1970s. "Taiyo ga Kureta Kisetsu"(Season of the Sun) was the theme for a yearlong high school drama "Tobidase! Seishun"(飛び出せ！青春...Break Out, Youth!) that was shown on TBS.
It has stuck in my head all these years since the song's arrangement is so anthemic, almost along the lines of a superhero serial, and also because it represents the sound of early 70s Japanese pop with the strings and orchestral brass, despite the fact that it's been labeled as one of the big folk songs of all time.
Here is the group consisting of Kumiko Nishiguchi（西口久美子）in the middle, and Shigeru Iwaku and Mari Takada（岩久茂・高田真理）. Aoi Sankaku Jougi first formed in 1971, and released 6 singles and 3 albums before their breakup in 1973 due to creative differences. "Taiyo", released in February 1972, was their only big hit, written by Keisuke Yamakawa（山川啓介） and composed by Taku Izumi（いずみたく）. The song hit the No. 1 spot and won the Newcomers Award at the Japan Record Awards while selling over a million copies.
I'd think people in their 50s back in Japan would be swooning about this one.