Sounds like many mothers would have been clueless about my "diaries" I'd buy or ask for a steno book for school...sometimes a comp book and it stayed mixed with my other comp books or notebooks in my backpack. The only time my mother ever ventured to read a steno book was when I subconciously left it laying in the middle of my perfectly clean floor and to her, that was like a big flashing sign that said "read me" The first five pages were about the secret I had about my uncle. She did not embarrass me but did show it to dad. Nothing was ever said until dad and I were driving to visit a college and he managed (idk how) to casually bring this uncle up and things went from there.
I thing teens deserve to have thier journals as thier own. If they want you to read it...it will be casually left out in the open as I did. but I mixed it with school work mostly and never had one that said "my dairy" on it. It's stupid they even make those.