The Church doesn’t insist on formal marriage classes, although it does offer Marriage Prep classes that can be exceptional, but instead it focuses on teaching principles from 18-month-old nursery onward that can make Christian disciples of its members - and that process creates people who can have happy, successful marriages.
According to the most recent research available, the general divorce rate among Christians varies from about 20-27%. The overall divorce rate for Mormons who marry outside of the temple is roughly the same. (For those who marry outside the faith, the divroce rate leaps to about 44%, below only those who live together prior to marriage - about 49%.) However, the divorce rate for Mormons who marry in the temple and remain active drops to about 6%-10%. Whether or not individual wards and stakes do enough to retain their youth can be debated separately, but for those who “complete” what the Church offers, the marriage preparation is exceptionally effective.