Mary Kay Letourneau and Husband/Former Student, Vili Fualaau, Separate

Mary Kay Letourneau -- the teacher who served more than 7 years in prison for having sex with her student, who she later married -- is about to be single again ... TMZ has learned.

Vili Fualaau filed for legal separation earlier this month.

Mary Kay was 34 years old in 1996 when she began having sex with her sixth grade student. They married in 2005.

At the time Mary Kay started the affair, she was married with 4 children.

Her story inspired the movies 'All-American Girl' and "That's My Boy."

Mary Kay and Vili have 2 kids of their own.