What constitutes being "old school" in the military?

Me

When you DEALT W/A E5 SSgt and thought he was the real SHIT

The line is where I say it is.
Now shut up and get back in ranks.

Riding up the hill with Teddy. A colt .45 auto in one hand and a sword in the other.
Shouting Yeepie, Kiy Yea, as your ride. But that would be Bruce Willis and not Joe Biden.

So that ever-moving line you asked about is somewhere between the examples above.

Joined the Marine Corps when they were still issuing brown shoes and leather gear along with a bottle of black shoe dye. The DI said make em black and we did. Leather boots with the rough side out which we wore in the field and spit shined in garrison. We kept our M-14's in rifle racks in the middle of the squad bay, secured with chains an a lock which we had a key. We could unlock our weapon at any time for cleaning or just personal drill practice. None went missing and no one was injured.

The term "old school" is whatever your relative time frame is - for me it goes back further than old school. We used to refer to ourselves as being products of the "brown boot army". Yep, back in the 50's that is what we wore. Later on they issued us bottles of black dye - first it was for our low quarters and about a year later our boots. Ike jackets back they too. A set of Corcoran jump boots and tailored OD's and you were pretty sharp - or at least we thought we were. Hated those greens when they first came out. Almost forgot, we also had about a 3 year period where we wore Bermuda shorts and pith hats during the summer (that was late 50's) Loved the TW's after I went to OCS in mid 60's during the Vietnam build up.

There will always be an "Old School" generation. Someone who served maybe 20 years ago could be Old School today.
Real old school for me was the following. Being issued brown leather boots, die them black and then add one full can of kiwi black polish. Spit shine with cotton balls until they truly shine. Old School was before Drill Sgts. They were TACK NCO's and physical contact with a trainee was customary. Wooden floors in WW ll troop barracks were highly polished and you did not walk on the floors unless you were wearing only socks. At night you had a fire guard insuring the coal fired furnace kept the barracks warm at night. Your issued M-1 Grand was kept in weapons racks located in the center of the floor. Often unlocked. You rode in WW ll duce and half's and jeeps. If you rode in a vehicle you were lucky. Otherwise, you ran or forced marched everywhere. Buddied up with shelter half's and slept with another guy in a single tent. Ate C and K rations....cold. Cooked, shaved and washed from your steel pot. If you were a Sgt. E-5 you were God. If you failed to salute an officer or within 50 feet of the U.S. flag, you might well begin looking for a shovel and begin digging a deep hole and crawl in.
Old school included a "Drumming Out" when someone disobeyed a direct order or equivalent breach
under the UCMJ. Anyone who has witnessed a Drumming Out will remember it for life. An old school custom still appropriate today.

It means you hate comfort and always try to make things as terrible as possible for people.

If your father was a member of the "brown shoe Army" and dyed his combat boots black, and you wore them during your active duty (hiding the brown eyelets), and still have them in your basement. Also, you wore khaki's during official summer, brown shirt with your "Greens" and a garrison cap. When "beanies" only belonged to Special Forces (and don't ever call them that around real snake-eaters). You qualified Expert on M-14 and understand what an M1905e1 is and where it goes. It was when KP wasn't done by contractors, and Unit Police patrolled your Kaserne, not hired security "rent-a-cops." (Oh, yeah, we had 4 maneuver divisions in WEST Germany). AND, when you couldn't trade them, ate Ham and Lima Beans C-rats out of a can, cold, because you were in black out conditions. And you keep your 1965 issue P38 on your key ring now, because you only a few months back stopped wearing your dog tags every day, where you kept them before. Then there's breakfast direct from the "Mermite Can" into your "mess kit" to accompany coffee boiled in your steel pot - can't do that with Kevlar.

Around 1987, Allan Bloom wrote a book entitled, "The Closing of the American Mind", which really heralded the time when PC (political correctness) really began to take shape. We still had a great many troops, grunts, aviators from the Vietnam War in the services, but they were leaving year by year. When that changed, along with the end of the '70s draftees, society had to coax people into the services with all manner of changes. Anybody who has served within the time periods from Vietnam and into the 80's and 90's knows precisely how different the training and the climate of operations works. It's not all bad, but it sure has taken a lot of bite from the fight. We need, and always will need warriors, more of a Bushido code of ethics, and a helluva lot less Dr. Spock, Hollyweird, and all this leftist feel-good drivel.