...and now I've read it again. It sais that "It's usually a good idea to have about one Journal activity per week." Nothing about any limitations. Exactly where in the system is this invisible limitation set? It would be really nice if the students at least got an error message that told them something like "Hey, you've written a lot, and maybe your teacher should've make another journal for you to write in by now..." 
More serious; the problem is that this limit isn't visible to any of the users (teachers nor students), and it doesn't produce any error messages when they write above it. Maybe in version 342 beta build 12.6565.3414? 