It gets to the point that any elf over a few hundred years old should be 20th level in whatever class they want.
Ahh yes, the elf problem...
The elf problem was introduced in 3e and has been a problem for fans of world building every since.
It's ultimately a result of the linear increase in XP required to obtain a new level. Back in 1e AD&D when I first began considering this issue, each level required twice as much XP to obtain as the prior level. So the cost of getting from level N to level N+1 was the same as the cost of getting to level N. That meant that for any constant rate of XP gain, that an elf with his binary 3 orders of magnitude longer life span would only get three levels higher than a human with the same approach to leveling up.
This was not a big problem. If humans could get to 2nd or 4th level, it wasn't that big of a problem that elves could get to 5th or 7th.
But once the needed XP goes linear, the elves binary 3 orders of magnitude longer life span just gets silly and it's not obvious what you do about it. As a kludge, I have elves learn more slowly from non-adventuring tasks, but it's really not the best solution or the one most easily justified from the fiction.
As for your particular method of getting XP, in my game it wouldn't work. You don't get XP from defeating conjured monsters. The XP from defeating conjured monsters comes from the XP of the caster. But you don't get adventuring XP at all from facing an opponent that isn't trying to win, and in this case the caster doing the summoning isn't trying to win. But even if he has a large tribe of kobolds available that he is pitting against the apprentice, that still doesn't get full adventuring XP because I adjust XP awards based on how the threat is enhanced by the environment. For example, you'd get more XP from facing a dire tiger in a tall grass marsh than you would facing the same dire tiger in an arena. But in this case, it's a false arena. It's being very carefully managed so as to not actually yield any real threat. And if you have a false arena where the threat is managed to the point there is no threat, I don't consider that adventuring at all. The wizard teacher has a slightly rigorous method of training apprentices, that's all.
On top of that issue of defining what adventuring XP is earned from, I use a different custom XP chart so that past a certain point the XP you get for a given CR is reduced. Even if the kobolds in the arena are well armed and given some sort of chance to earn the crits fairly, and the tribe is cunning and trying to train up volunteers so that the wizard doesn't have full control over the CR of the encounter, and the wizard teacher is the sort that willingly culls the weak and would stand back and let the kobold kill the apprentice (that point where the kobold rolls 2 20's and the apprentice rolls 2 1's), and I decide this is true risk, there comes a point where killing kobolds teaches you less and less and eventually nothing.
That still in and of itself doesn't solve the elf problem though.