When you go to school tomorrow, don't forget to mail this letter.
Literal
Tomorrow school [to-に] go time, this letter [object-を] post [to-に] put [nominalizer-の] [object-を] forget-not [so-that-ように] [confirmation-ね].
~時 ('when/at the time of') creates a temporal clause. ポストに入れる ('put in the mailbox') is the concrete action of mailing. 忘れないようにね combines the admonition ~ないように with the softening ね. A naturally layered everyday reminder.