you are nit-picking the content of what she said while ignoring the meaning.
"This place sucks. I came here to learn to control my powers--not use them to fight crazy dudes every week. I'm not involuntarily falling through floors like I was. I don't know if I need this place andymore. I certainly don't like it."
She DID join the school to learn to control her powers, the adventure seemed exciting and fun at first, but she was here primarily so she wouldn't accidentally fall through the floor on any given occasion. Once she started to figure out her powers, i'm sure she loved the celebrity of it, but at the same time... Right after she joined the team she was with Wolverine when he left Cyclops to die... her first "mission" was with a known and feared killer who, as she soon found out, actually tried to kill one of her partners. then she became an outlaw, and was hunted down by both Magneto and the Ultimates. Then, one of her teammates died. And unlike in 616, she has no reason to suspect beast will come back to life...Then a Serial Killing scientist broke into the school trying to kill them... They were hunted as prey on a subsaharan african island by a TV mogol...Some crazy twin businessmen tried to abduct one of her teammates...Then Magneto, with his homicidal grudge against the X-Men, almost escaped again (she doesnt know its not the real magneto in captivity)... ANY ONE OF THOSE THINGS would be reason enough for a young girl (younger than the rest of her teammates, you have to remember) to decide that DESPITE THE FACT that she wanted some fun superhero adventures at first, that this was far too much for her.
Most of all she's trying to convince herself to leave, and she's rationalizing ths situation, like LOTS of teenagers do. She's loved aspects of the school, but she can't take it anymore
Like, imagine a teenager dropping an ice cream, and then saying to herself, "**** it. I don't even like ice cream all that much anyways."
You are paying too much attention to the wording and not enough to the meaning.
I think this is one of the most interesting things they could have done with Kitty, whom I think many writers didnt know what to do with.
EDIT: Just checked... She was 14 when she joined the team. Somehow I think that needs to be taken into account.