Meaning of the expression "old school"

Yes, basically you hit the mark.

Based on Webster's Dictionary: (Noun) Adherents of traditional policies and practices

According to Vocabulary.com: The old school is any group of people who are traditionalists: they do things the way they've been done in the past.

Old school means something close to old-fashioned, but it’s a term with more pride behind it. If someone says, “I’m old school,” they’re saying they do it like it used to be done, which they believe was a better way. In mob movies, the old school usually thinks they’re better than the younger criminals. An old school teacher might prefer not to use computers in class. This word can be used as a noun or as an adjective, and the opposite is new school.(it's rarely used).

Short Definition: A class of people favoring traditional ideas.

Example: The title of the Los Angeles Times dated on 2015-12-13

J.J. Abrams on-going old school and how 'Star Wars' differs from 'Star Trek' It tracked back to J.J. Abrams' experience in 1977 where his fan's cred to "Star Wars" franchise is unquestionable. And now, he is given a big responsibility to "take" this newest "Star Wars" movies to a new height.

"Now he finds himself holding the keys to arguably the biggest pop culture franchise on the planet. With "The Force Awakens" hitting theaters Dec. 18, millions of "Star Wars" fans are hoping the Force is with him but the stakes are unimaginably high. It's "do or do not" time. There is no try."

So, we can conclude that Abrams need to combine his 'old school' ways to cater the 'new school' generation that might only know Star Wars Movies just recently.

Source: http://www.latimes.com/entertainment/movies/la-et-mn-star-wars-analysis-20151216-story.html

Other example:

1.Old school teacher teaches using blackboard and chalk. S/he explains methodically and mentions the lesson one-by-one. S/he uses one-way communication.

2.New school teacher teaches using high-technology gadget and software to help s/he explains the lesson to the class. S/he often creates class discussion, and games to engage with the students. S/he uses bi-directional communication.

[This is from my experience] When old school word is conjured up, I usually thought of traditional & conventional people/custom/habits.

Hope this helps :).

"Old school" is usually used as an adjective: "That's so old school." (I've never heard it used like "In the old school...") And it describes a thing that is old fashioned. Not necessarily from, as you say, the old time. Some things or expressions from old times might not be considered old school.