In what way are temporary promotions still done? I think we might be talking about different things here. If you’re talking about frocking, that’s not the old-style temporary promotion, where you’d get the pay and benefits, as well as the UCMJ authority, of the temporary grade.
A college degree, screening, and successful completion of Officer Candidate School would do it. OCS has various courses. stankow’s part about going to Basic Training first is not needed for the Marines.
From http://www.marineofficer.com/eligibility/elig_index.html
If you already have a degree, as Zev does, the Officers Candidate Course is a 10 week course in Quantico, VA you can drop out of if you do not like, and if you complete it, you are given the CHOICE to accept a commission as a Second Lieutenant, USMC. Or you can walk away.
Is the old Marine PLC program still in existence? I didn’t see it on UncleBill’s link, but I could have just missed it.
Click the FAQ, and it is listed. I don’t kinow if they still do PLC Junior and PLC Senior classes (split OCS over two summers, 6 weeks each) AND the PLC Combined (10 weeks, one summer), but the program exists.