Can my parents claim me on their taxes if I was 18 and in high school part of the year, but now I'm in the military?

They can claim you if you did not provide more than half of your own support for the year. 

TO help make it more clear;

Your parents qualify to claim you as a dependent on their tax return if you did not provide more than 50% of your own support for the "entire" tax year.  Having started boot camp in September, I doubt it was even possible for you to do that.