Mia's are as well. Hmm... There are two places that we could use to level, The Battle company (which wouldn't end well) and the LoL Headquarters (At least, I think you can gain levels from this).
They could always go to Mia's house though. Or, better yet, they could (try) sneak into the Casino because I don't know. (Actually, they might be able to gain a few level's by beating up any bouncers/security guards Pokemon in there).
Or we could just make something up (e.g a random patch of grass, an abandoned warehouse etc).
And then there's the Tularosa Research Facility, but I don't know how that could help with levelling (unless the professor were to give out Steroids Rare Candies).