It seems that students can edit journals after the closing date of the Journal!
If a student clicks on the "Journals" link on the Activities toolbar (or go one breadcrumb out of any Journal), that student can see all of their journal entries. It was just pointed out to me that there is also an "edit" link on each one. Even the closed ones!!
I logged in as a student and edited a closed journal. It took the edits (but it also did update the last edit time). I don't think the students should be able to edit after the closing date.