As I mentioned long ago in the article for Akai Tori's（赤い鳥）"Tsubasa wo Kudasai"（翼をください）, when I was teaching on the JET Programme in Gunma, one of the major events during a junior high school year was the annual school chorus competitions. During one of the two years I was there, I went with the teachers and students of one grade for the day to watch our choruses compete against their counterparts from other schools in the prefecture through songs that had been practiced over and over again daily. "Tsubasa wo Kudasai" was one of the mainstays as a go-to chorus song along with "American Feeling", originally by Circus, and then "Die Moldau" as seen above. "Die Moldau" is actually the German name for the original "Vltava" as one of 6 symphonic poems in the set known as "Má vlast" by Czech composer Bedřich Smetana.
In the years since my time in Gunma (1989-1991), there probably have been multiple additions to the chorus song slate. One that has been added is singer-songwriter Angela Aki's（アンジェラ・アキ）"Haikei Jūgo no Kimi e" (Letter: Greetings to a 15 Year Old) that was her 8th single from September 2008. And as you can hear in the video, it's a perfect fit. According to the Wikipedia article for the song, her single was a self-cover for the original version that had been created by Aki at NHK's request for a 2008 national school chorus contest that the national broadcaster was sponsoring.
The whole premise behind "Tegami" is a communication between a teenager and his/her much older self during which the latter provides some of the necessary reassurances and wisdom that have been gained during the time difference. It is almost like a reverse time capsule to lovingly benefit the young student.
"Tegami" became Aki's most successful hit to date, selling over 210,000 copies and going Platinum. It reached No. 3 on the Oricon weeklies, becoming the 45th-ranked single of 2008 and then hanging around to become the 71st-ranked single of 2009. Not only has it become a part of the chorus competition lineup but it's also become popular as a ballad to be performed at graduation ceremonies (dry eyes would probably be fairly rare here).
Continuing with the NHK connection, the song also became part of the "Minna no Uta"（みんなのうた）lineup although I couldn't find that particular video, and Aki was able to perform "Tegami" in consecutive Kohaku Utagassen broadcasts in 2008 and 2009. In total, she appeared on the NHK special 6 times. It has also ended up in a few commercials as well.