中学生 why not secondary student life?
中學生活 does mean 'secondary school life', 中學生 however must mean 'secondary school student'.

生 is a compound ideogram comprising 屮 (sprout) and 一 (ground). A sprout emerging from the ground therefore symbolises life and growth.

Besides meaning life (as in 生命), living (as in 生活), growth (as in 生長) etc., 生 has a few other noteworthy derived meanings. 生 in 中學生 corresponds to the second row:






Definition
Example




n. the living
養育眾生 lit. to feed everyone that is alive


n. the literati, the learned, pupil
學生, 生徒 (retained in Japanese), 諸生 (obsolete)


n. nature; fundamental character (now replaced by 性)
天性 (天生 in classical Chinese)


adj. raw
生魚片 lit. raw fish slices





The above is an incomplete list, only there to give you an impression of how diverse in meaning 生 can be. I also found a few remarks online that suggest pupils are like sprouts, as they 'grow' and 'absorb knowledge'; reasonable as they may seem, I however do not know how etymologically sound this justification is, as 生 has a long tradition of being a laudatory title for the literati in general (子 as well, as in 孔子 Confucius, 孟子 Mencius; surely one cannot interpret it as 'son'). But if this is good for memorisation, then by all means!