Sweaty hands usually means you are anxious. You are probably still in high school, which means dealing with tons of pressure. Plus you have a girlfriend as well, which means you have more things to worry about.
Let’s look at it this way even if you claim you are not nervous. Anxiety can come in different forms, some even subconsciously. In this case, you are anxious about your life, then you get sweaty palms, and next you are worried about the sweaty palms so the circle continues. Well, that is one case anyway.
Aside from that, there are hormonal issues. It doesn’t sound that serious. It’s just puberty messing around, like how some teens will have their faces riddled with pimples, and for you, it’s sweaty hands instead.
However, do take note that there is a condition called “hyperhidrosis” which cause various body parts to sweat profusely, sort of due to your nervous system overreacting to the outside. Coincidently, the condition starts around the age of puberty and triggered by external factors, like stress, spicy food, etc. So you might be suffering from that, although a relatively small chance of it (only 1% of Americans has it).
Make an appointment with your GP if it continues. It’s not life-threatening in any way, except to your social life, treatment is usually considered to be for quality of life. Doctors can fix you up quite easily; most treatment takes a long time to complete, however.