What constitutes being "old school" in the military?

"Old "School" is indeed a relative term. When I joined the Navy in 1962 there were sailors who served during WWII still serving. I met one who was in the Bataan march. These were truly 'Old School" sailors who couldn't believe how easy we had it. At least two generations of sailors have passed since I was discharged in 1966, so I figure I would be "Ancient School" today.

After reading most of the comments, the one thing I did not see was Spec 5, 6, & 7. I saw someone post acting jack though. I did not personally see a Spec 6 or 7, but I did see a Spec 5.

Pealing potatoes in mess hall, Duce and half driver, nonconforms got the grease pit, WWII barracks, coal fired tent heaters, C-rations, M-14 rifle,

Nowadays simply having discipline is considered old school.

You have a 20 year letter and you're still serving!

I just found out that my 3rd Great Grandfather, enlisted in the US Marines in 1811. Long before I went in in 1961. He was "Old Corps"

SPC4, SPC5, and SPC6's in the Army.

Big collar BDU's with OD Green Field Jackets.

When you show up to formation like this. You have gone as old school as you can.

Old school for me would be before the year 2000, and the age of electronics, when the fast computer a soldier carried was his brain, instead of his cell phone.