我一写完日记就上床睡觉。 this sentence is the grammar I mentioned in my comment, using 一⋯⋯就⋯⋯ to show something is right after another. In this case I would say an english translation could be: I go to bed right after I write in my journal.
下课了我就去看电影。The 了 is a separate grammar point here. 下课 is a verb to get out of class (cause its over) and 了 is serving its common purpose after a verb to show completion. The 就 here means as soon as. So you could translate the sentence: As soon as I am out of class I go to see the movie.
So the two formats you mentioned aren't directly related, and it may not be useful to compare them too closely. If we were to compare them anyway, I would say the first sentence was worded to show "do action one1 then immediately action 2". The second sentence was worded to show "at that time immediately do action 1".
link to a wiki page on the 一⋯⋯就⋯⋯ structure for you. The same page should have links to a variety of other 就 uses as well: