old school vs stratum

old school and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
old school	Yes	No	No	No
stratum	Yes	No	No	No

As nouns, stratum is a hypernym of old school; that is, stratum is a word with a broader meaning than old school:

Other hypernyms of old school include class, social class, socio-economic class.

old school (noun)	stratum (noun)
a class of people favoring traditional ideas	an abstract place usually conceived as having depth
	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)