The training event will involve six instructors leading 40 armed volunteers through a simulated shooting on Saturday, with teenagers acting as students, the sheriff's office said in a statement.
Seagal will lead the training run, focusing on various aspects of shooting scene management, including "entry room tactics and hand-to-hand tactics," according to Arpaio's office.
Arpaio, known for his tough stance on illegal immigration, dispatched members of his volunteer posse to patrol schools last month in the wake of the gun rampage that killed 20 children and six adults at a Connecticut elementary school in December.
Those murders touched off a renewed debate over gun violence in America and prompted National Rifle Association head Wayne LaPierre to advocate placing armed security guards at schools.
President Barack Obama has proposed the most sweeping package of gun-control measures in generations, calling for a ban on assault weapons and other steps likely to face a tough ride in the Republican-controlled House of Representatives.
The Arizona event will take place at a school campus in the Phoenix suburb of Fountain Hills. Semi-automatic weapons and handguns firing marking rounds will be used.
It will not be Seagal's first foray into law enforcement. In 2011, he was sworn in as a sheriff's deputy in Texas' Hudspeth County, which runs along the Rio Grande, after expressing an interest in patrolling the border. He also appeared in a reality show detailing his work as a reserve deputy in New Orleans.
Seagal, a resident of the Phoenix Valley and member of Arpaio's posse, starred in big-budget films in the 1980s and early 1990s, earning a reputation as an action star in movies like "Above the Law" and "Under Siege."
In the last decade, he has appeared mainly in direct-to-DVD, low-budget films while working in law enforcement. His last role was as a corrupt Mexican drug lord in the Robert Rodriguez grindhouse flick "Machete."
Arpaio's 3,450-strong posse of unpaid men and women has for years helped the sheriff target drunk drivers and illegal immigrants, and chase down fathers behind on child support.
Last year, Arpaio sent posse members to Hawaii to investigate the authenticity of Obama's birth certificate at the request of local Tea Party activists - a key Arpaio constituency.
