why did he work in prison as a teacher

The film tells us he got 6 months community service for the crime of embezzlement (the $10k he stole from his company to pay poor Winston). He used to be a teacher, which his wife mentions earlier in the film ("We can't pay for Amy's drugs on two teacher salaries"). If you listen to Charles's discussion in the final climax in the prison laundry room, he says he CHOSE that particular assignment...it actually took me a second too, but obviously Charles chose the assignment because he became aware there was someone at that prison he very much wanted to "talk to." You follow me?

thanks this makes a lot of sense. one more thing i don't understand is that did he tell his wife the truth or he just told her that he was being blackmailed for money and never mentioned the affair?