Prison ministry is not a place to practice but to sharpen your people and your message.
Incarcerated men and women are hungry for God. Maybe for the first time they are sober, alert and ready for the Good News of Jesus Christ. I've seen firsthand, inmates frustrated and never return to a church service because they cannot relate to or understand the speaker.
I was in a prison where the speaker didn't know a common childhood Bible story. She had a heavy accent and was very difficult to understand. If the teacher has substantially less Bible knowledge than the inmates, THIS IS NOT the place for him or her.
On the other extreme, I have heard speakers talk way over the inmate's heads. Using theological terms without explanation - terms such as "propitiation, sanctification, justification" - these are important terms if we explain them. We only have a short amount of time to concentrate our words to make an impact in their lives. If you don't use these terms everyday, ask yourself if it is necessary? Then I'd advise you thoroughly explain it as you teach. Keep the message simple, easy to understand, with scripture broken down and explained.