AFAIK, you enter the US military as either an officer or an enlisted soldier.

No enlisted soldier "becomes" an officer. Not as a promotion, at least.

You have to go to school to be an officer. Most officers enter the military as an officer.
There are paths to become an officer if you are an enlisted soldier, but they're arguably harder than an 18-yo joining ROTC (Reserved Officer Training Corps - a college/military hybrid program which upon completing college, you enter the military as an officer.)

Joining ROTC isn't really that hard to do, but moving up from enlisted to officer requires you to be considered exceptional at your job by your commanding officers, and to be recommended for officer school, then to actually go to what amounts to college to become an officer.

It's do-able, but it's not the common path.