If you are enrolled in school, and then incarcerated is it legal for the school to start collecting?

My fiance was enrolled in a tech school, and he went for almost a year and then he was incarcerated for six months, immediately after he went in, me(his wife) and my mother contacted the school and let them know that he would not be attending school from three to six months, and they told us that everything would be ok. I had him sign a paper that said we could get his school records, now hes home, trying to renroll and they are telling him he has to pay 1300$ to start cles in a week, bc for the past six months they have been calling and mailing out to the wrong address, so we had no notifications that they were collecting, not even from the school that we contacted. Is that legal? Like in the eyes of the government he just dissappeared for a minute, they dropped him from the state insurance the week after he went in so why should he be responsible for his debt if the state wont even insure him. But anyway, just is it legal what they are doing, what can we do to better the situation?!