English and ESL Instructor
Whenever you use "one of...", you always need to pluralize the noun that follows the expression.
Incorrect: One of my classmate
Correct: One of my classmates
However, for the verb that follows the noun, you are supposed to use the same conjugation as for the pronouns, "he", "she", or "it":
Incorrect: One of my classmates are from Russia.
Correct: One of my classmates is from Russia.
If you had to choose a subject pronoun for "one of my classmates", then it would be "he" or she". In that case, the sentence would read as either, "He is from Russia" or "She is from Russia".
Remember these rules when you are writing from now on.