When the boy turned 9, in September 1899, his loving mother enrolled eight-year-old Osip in the St. Petersburg commercial Tenishev School, where the tuition was quite high. She did this despite the financial difficulties that began to plague the Mandelstam family by the late 1890s. The Tenishev School, located at 33 Mokhovaya Street, was known as a Russian forge of “cultural personnel” of the early 20th century; besides Mandelstam, Vladimir Nabokov later studied there as well. This “new type of school,” established on the initiative of Prince Tenishev, became the first educational institution for young Osip. The school’s archives even preserve the first mentions of him as a first-grade student: “A smart and capable boy, but very proud; speaks Russian excellently, always told stories coherently and literarily. Knows the history of the Old Testament well.” It was at this school that Mandelstam made his first public appearance—in one of the evenings of 1907, he read his poem “The Chariot,” unfortunately long lost. It is no surprise that the poet often compared this institution to the Tsarskoye Selo Lyceum—like Pushkin, his poetic gift blossomed in his youth. He graduated in 1907. As Osip later recalled: “And yet there were good boys at Tenishev. Made of the same flesh, the same bone as the children in Serov’s portraits. Little ascetics, monks in their childhood monastery, where in notebooks, instruments, glass vials, and German books there was more spirituality and inner order than in the lives of adults.”

Sources:

https://ria.ru/20160115/1359081640.html

http://www.cityspb.ru/blog-748587/0/

https://ru.wikipedia.org/wiki/Mandelstam,_Osip_Emilevich