TOPIC: Class of Events - Prerequisites - how to do this?

My client runs a series of different classes are like a 101, 201 & 301 series of classes. That is, 101 is a prerequisite for 201 which is prerequisite for 301.
These classes may be run at any time of the year and can be run multiple times in any year.

My client only needs to track if the person has done 101, at any time, to be eligible, to do 201. And, how can DT Register stop a registration for someone that has only done 101 but is trying to register for 301?

How can DT Register deal with this? I get it that there's prerequisite events in DT Register but, from what I understand right now, it is to a specific event and not a "class" of events.

Thanks,
Andrew