Florian, sorry for the lack of response earlier; we browse the forums as we're able on top of our other duties, but if you need a guaranteed response, emailing our
support ninjas is the way to go.
That said, based on the description of what you're trying to do, it seems like a weekly recurring item in OmniFocus would cover this. Create a task with a start date after the end of the previous class and a due date that's before the next one, then set it to repeat every week.
Something more sophisticated might be possible, but you'd need to write some AppleScript to move the information. Once the event that represented the previous class was over, iCal would trigger the script to create the item in OmniFocus...