Meh, we had the same thing.
It's just a way for teachers to make sure you're keeping up to date, it's in your own interest and there is no reason why it shouldn't be legal.......it's not like the teacher can't just go "well hypothetical speaking i'd give that a 10/10" and that's perfectly legal if they don't ever give you that mark, so it's really stupid to begin with. I guess some teachers see how stupid it is and choose to ignore the rules and use parts of your MDP as some of your assessments.
You're not getting ripped, it makes no difference really, you are quite able to walk up to your teacher and ask them directly what they think of your MDP and what you have to change to get full marks.