AIO Discovered my coworkers look through my lunch box,

My 22f coworkers range from 26 to 50's (men and women). I started teaching in a secondary school this January. When I started I was told the office provides food, I tried it out for a week and decided it wasn't for me so I decided to be making my own food at home.

I usually pack food in a lunch box with either leftovers or something I make the morning off and a few snacks (I'm the only one who packs my own food). Today I just discovered that my coworkers (the six I share an office with, and sometimes others from other offices) look through my box and check what I'm eating on that day whenever I am in class. And since my subject is one of the key subjects I'm usually out of the office for a longer period of time hence they have time to do their thing (look through my lunch box).

One of the teachers whom I thought was my friend told me it has been happening for weeks. He even admitted to doing it with the other teachers. Now I'm just really angry and feel like they violated my privacy. Will I be overreacting if I tell the principal about this? Or what should I do. Any advice is welcome.