rule "room conflict"
when
Lesson($id : id, $timeslot : timeslot , $room : room)
Lesson(id != $id, timeslot == $timeslot , room == $room)
then
scoreHolder.addHardConstraintMatch(kcontext, -1);
end
You don't.
As Lukáš indicates in the comments, OptaPlanner has no DMN support.
On the other hand, regular open-source Drools does have DMN support, but no built-in concept of constraints.