My parents won't let me change my name for my new school, and I was wondering if anyone knew anyone masculine versions or nicknames of the Kirsten (except Christian/Kristian, for personal reasons).

I would just use Kris really. Hard to think of others. Maybe Krister? I have a Latvian friend who has that name.

...Stan? K (just the letter, or Kay)? If you have a cool last name maybe go by that?

I agree, Kris or K/Kay.  Both more gender-neutral than masculine, but at least not super-femme. 

Though, you can ask your friends to call you whatever you like.  I had a friend whose name was Kelly, but she went by Deirdre or Dee from high school on. 

alter your middle name into a guy nickname?

Either that or I found this list http://www.buzzle.com/articles/nicknames-for-guys.html
Captain Cheesepants caught my attention 

Kei


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How about "Kurse"...?

I have a K legal name too and I've always been called "Kit"

Yeah, I think Kris will most likely be your best bet or maybe Kirk. Really depends on how you feel about the name. Hopefully you find something that suits you though.

Your parents seem to not want you to transition. Changing schools is a good time to do so, you should talk to them and stand your ground about being male at that new school.

Maybe Karsten or Kar?

Kir/Kier, maybe?

King?