Gabi,
Your desire to be modest is very biblical and God-honoring, and honoring to the young men around you.
As a far as pant vs. dresses...the Bible simply does not address it. In Biblical days everyone wore robes, men and women...but there were definitely differences between the two. The Bible does speak in the old testament about women not dressing like men, but we simply don't know what that meant back then. The point for today would simply be:
There is no biblical restriction against pants, or even modest-length shorts for women...as long as you look like a woman wearing pants, not a man wearing pants.
In Scotland, men may wear kilts occasionally (hundreds of years ago they wore them all the time)...but no one would confuse a man in a kilt for a woman in a skirt.
Just be sure that you respect the sufficiency of Scripture enough that you don't make lists of sins that God did not call sins. If God wanted to forbid women wearing pants, he could have, but he didn't. It is dangerous to start calling things sins that God doesn't.