Thanks, nice to see appreciation from a pro.
About the ensemble. I used to go to some arts' school, there was a folk song circle there. They didn't perform anywhere, still they were called an "ensemble". It may be just some informal expression. The definition of the word has changed a bit since the times of the USSR. Nowadays, nobody calls a brand new band an "ensemble", except for folk groups. But I agree, "ensemble" sounds more funny, I'll edit this.
Bayan is a Russian button accordion, I'll add the footnote.
I'm not really sure about Lenochka. I think, she is just younger than the other members of the band, and if they, for example, study in the 11th form, she is in the 10th.
Now the school system. In Russia, "младшие классы" are the grades from the 1st to the 4th, where kids of 7-11 years old study.
(1) Вокально-инструментальный ансамбль, "vocal – instrumental ensemble", was the official name applied to pop and rock bands in the Soviet Union.
(2) Such eccentric, yet simple names were common in the USSR, when all types of art had to meet the state's ideology.
(3) I've shortened the sentence. "Rhythm guitarist, lead guitarist" etc. would be too long and distracting.
(4) In the USSR "Ionica" was a common noun for an electronic keyboard (organ).
(5) Bayan is a Russian button accordion.
(6) In the original text the geography teacher is a woman.
In Russia, all the pupils of one school, from age 7 to 18, study in the same building.