'in my school days' vs 'during my school days' vs 'on my school days'

[Zsam_Lam]

i have doubt on the usage of the word

For eg:

1. the lecture discuss, the lecture states

2 a. in my school days, i went to picnic
b. during my school days, i went to picnic
c. on my school days, i went to picnic

3 a. it is not necessary to have more than one pen in pocket
b. it is unnecessary thing to have more than one pen in pocket

[Gray]