Using Serializable To Write Everything To A File At Once?

For a class, I'm making a prison system program. The prison has cell blocks, each cell block has cells, each cell has bed(s), and each bed has prisoners. There are also visitors and visits. I thought, rather than write all of those to different files, I could just write the entire prison to a file and make all the classes under it implement serializable. The instructor said that is a bad practice and we shouldn't do it. So I'm thinking I'll make the prison non-serializable. But the prison has maps of cell blocks, inmates, visitors, and visits. I think I'll make all those serializable and I'll also make everything a cell block contains serializable as well. Then I can just read the cellblocks from the file. But is that too easy as well? Is that a bad practice?