Immediately swapping out a level pet without passing or attacking will net them 0 xp. If you managed to get any xp and you did that, it was because your active pet died--not because you had the pet in. You'd get the same effect by just passing with the active (non-level) pet on turn 1. It's the same concept as Explode strats. (edited)

This comment is old, but I can assure you it is incorrect, I tried this today and got 675 rep for my pet with the hat on and it did not hit or do anything, immediately swapped after first turn to next pet.

You are conflating two different things. It is easy to mistake them; I've seen the same pattern in a comment on Mega Bite, where Ikky usually dies at the same time as the opposing turtle.

Dragons is correct, and gave the correct explanation for the XP. A level pet in the first slot that is IMMEDIATELY swapped out without attacking or passsing does not qualify on that basis for XP.

HOWEVER, when one of your pets dies simultaneously with the last opposing pet, any remaining pets in your team come forward to split the XP, and when there is only one level pet, it ofc gets all the XP.

This principle is used in Explode strats, where your second pet chooses to Explode when it will hill the last remaining opposing pet, and itself, leaving your level pet to come in and claim the XP.

It also usually happens in Mega Bite, where Ikky kills the turtle just in time for thr Whirlpool to kill Ikky. Your remaining pet(s) come in and claim the XP.

I just tried this strat 5 times, with a Level 1 in the first slot. In all 5, the Micro-Sentry died at the same time as Nightfang. If you don't have a level pet in the first slot, the Micro-Sentry will usually survive the battle, but with the wasted round, it dies simultaneously. And then your level pet comes in and gets XP. So my level pet got XP on all 5.

I then started with a Level 1, immediately switched to a Stormwing, who softened up Nightfang and died, and then brought in the Micro-Sentry, which finished off Nightfang without taking much damage. The result was that the level pet got no XP.

So, yes, you could use this as a levelling strat, but the reason for the XP is that the level pet comes in at the end, not that it earns it at the start,

As we have so many stressful events in WoW, I enjoy an Easy for a changer very much!