What are subjected in mathematics, geography, biology, language, music, etc. are forms.
In school each useful genre of forms is taught as a subject.
When a form is subjected, a theory (logical system) is implicitely or explicitely subjected.
Cf. When a word is used, a language is used.
We have been arranging formsto theories (logical systems) in our long history of human beings.
In general, teaching a subject is teaching a logical system of forms.
The reason why school subjects which are apparently different one another are of the same kind, called "subject", is that what is practiced in each is teaching a logical system of forms.
The difference of school subjects is the difference of systems of forms taught in school subjects.
Examples :
"Fan form"
<--
Geography
"Amphibian"
<--
Biology (Zoology)
"Chord"
<--
Music
"Parallelogram"
<--
Mathematics
Learning a school subjects :
Meeting forms, becoming familiar with them, and mastering effective ways of using them.