Jennifer Garner -- Crashes School Dance (VIDEO)

Jennifer Garner turned a mother/daughter dance into a fangirl sesh.

Garner was filming "The Tribes of Palos Verdes" at Friendly Hills Country Club in Whittier, CA Friday night. The club was also hosting a mother/daughter dance for Grazide Elementary.

Producers gently asked the hoofers to keep it down during shooting, so the make good was Garner making a surprise appearance, taking the mic and talking to the group.

Garner told the kids they probably wouldn't be allowed to see her movie ... it's about a wife who goes nuts when she suspects her husband is cheating.