In theory you can get to lvl20 that way (according to more than once source, it has actually been done). Once the Charr don't give you xp anymore, you start training (make them chase you) back to the rez shrine. Let them kill you until they level up. Once they level up, you will be able to get XP from killing them. Once you get close to lvl20, you will probably need to leave the game running afk for hours at a time to let the charr level up to at least 15 so you get the xp from them.
If you plan on doing this though, wait until you get to about lvl 19 before doing any of the quests, you still get the same xp for the quest, but you won't need to wait as long for the charr to level up to get the last little bit to lvl 20. Once you dominate in the academy, you should be able to solo right through the shiverpeaks.
I've been hearing about this strategy for presear level-up for a while, but I have to say that I have never felt the desire to actually do it. Other than the novelty of having a lvl20 presear, not sure what good it would do. The down side for me is that you won't have many skills and you will already be leveled out of the Ascalon Arena as soon as you get there.