Final answer:
The S and T lines do not specifically refer to certain grade levels for students; they appear to be part of examples that do not specify target audiences by grade. These could be part of a coding system within educational materials, and without additional context, their meaning remains unclear.
Step-by-step explanation:
The S and T lines mentioned in the question do not refer specifically to divisions within the educational system for junior and senior high school students. Instead, it appears to be related to examples within a textbook or study materials that do not directly specify the targeted audience by grade level. Historical context suggests the 'S' and 'T' could be part of a coding or naming system within educational materials or examples used within those materials. For instance, during the 1980s, Gay-Straight Alliances started to be organized by high school and junior high students, but this example doesn't necessarily relate to the 'S and T lines' unless further context is provided.
Similarly, the examples given about types of sampling and book costs for part-time students do not specify grade levels, indicating that educational material can cover a wide range of student experiences.