Rather then have more penalty for DPs, just limit XP by a simple level-based formula like:
You only get XP from mobs that are level (2 x your AR)+10 or less. So a raw newbie could 'leech' from groups killing levels 1-12, and by AR 55 you can get XP from mobs up to level 120.
Since this is AR based, people levelling yet another school to 100 will not be affected too much -- level 1 AR 35 could get XP from mobs up to 80.