Lane Garrison -- Teaching Prisoners a Lesson

Lane Garrison is putting on a real show in jail -- TMZ has learned, the former "Prison Break" star is teaching acting classes ... to his fellow prisoners.

Sources tell TMZ, the L.A. County Sheriff has Lane mentoring about 20 prisoners as part of the "Merit" program -- designed to teach marketable real-world skills to inmates.

In addition to acting, we're told Lane is also giving a crash course in screenwriting as part of the program.

And get this -- Lane's so moved by the experience, we're told he's promised the Sheriff to continue to mentor the prisoners even after he's been released ... whenever that is.

Lane's currently in custody after a domestic violence arrest last month. He's stuck in L.A. County Jail on a parole hold, stemming from his 2007 vehicular manslaughter conviction.