Is it illegal in Utah to move out when you are 18 if you're still in high school?
No, Utah's compulsory education law is 18, with no stipulations as to the person having to finish their schooling (although doing so would be the best choice).
Source nhunting.com. slingshots are illegal for shipping and can't be shipped either into or from Salt Lake City Utah. In slingshotforums.com its stated its legal to own slingshots in Utah with the exception of Salt Lake City Utah. Maybe its a Mormon thing. List was updated 12/27/09 so I can't tell if this is still true.