25 or 26 it doesn't matter, just be the fastest bald headed freak in a pair of silk shorts that you can be, that's all that matters. Don't go there injured, distracted, or unable to blow the doors off the PFT.
As for studying. 2.0 and go man. Enjoy your time while you can, cause once you are in life is going to change big time. I will promise you that nobody is ever going to ask you what your GPA is ever again unless you do something nutso like NPGS, TPS or heaven forbid MOI.
The real question is the one you haven't asked yet. The real question you have to ask yourself, is do you like your balls sweaty hot and nasty, or frozen, and prune like? If you like hot and nasty, then go for the June class and watch them rott off your body. If you like it cold, then go for the December class and bring a lot of handwarmers to bring the boys back to life.
FLASHBACK Winter 1997: "Balls, meet Mr. Quigly. Mr. Quigly, meet my balls."
If I was you I'd go in the summer and suffer and like it, who knows you might even get the prestigious silver bullet award and some free books from the Marine Corps Association .
Are you for real?