Adventures of Petrov and Vasechkin, Both Usual and Unbelieveable (1983)

Best friends and mischievous third-graders Vasya Petrov and Petya Vasechkin, despite their differences, share a love for adventures. Romantic Vasya falls for his serious, excellent classmate Masha and writes her an anonymous love note. However, the quick-witted Masha identifies Vasya due to his errors and rejects him harshly. Upset, Vasya confides in Petya, who then devises a cunning plan to win Masha over.