How do teachers of lemudei kodesh (Judaic studies) and rebbeim decide on what to teach their students?
There are two criteria for secular studies regarding whether a particular subject should studied or taught in a given curriculum:
Theoretical consideration: The subject should be a body of fundamental knowledge which is a prerequisite for the understanding of other subjects.
Practical consideration: The subject should have a practical significance in the sense that its understanding would be useful in taking efficient decisions in some of the practical problems.
Obviously Torah study is different, but in particular how does Judaism (or longstanding Jewish practice) indicate that we should decide what is more important to teach and what is less important to teach in the world of Torah education?