Looking for the right word--no joke

It's right there in the back of my mind what the word is I'm looking for, but I'm not able to retrieve it. Here's the sentence, can you help me fill in the blank?

Every doctoral student who received their degree from Judith, and others who knew Judith well, were conspirers in perpetuating a long-running ______ of calling Judith by her childhood name, “Eddie.”

The word "joke" is not what I'm looking for. What I want is more about a deep sense of close familiarity only known to a few people. "Tradition" isn't right either.

 

This thread suggests 'shibboleth' which is ... interesting, at least.

All I can think of would be private or inside joke or potentially gag. In the vein of 'tradition', maybe ritual, custom? I'm not sure if there is a word for the specific thing you're wanting one for.

 

Gag?

 

Rite?

Kind of in the same vein as "tradition," although with a bit of religious bent.

 

Jest

 

How about:

Every doctoral student who received their degree from Judith, and others who knew Judith well, all conspired to perpetuate the custom of calling Judith by her childhood name, “Eddie.”

 

Habit. Practice. Ritual. Custom.

I could've sworn I'd have more, but that's all I came up with.

 

How about rewording the sentence:

Every doctoral student who received their degree from Judith, and others who knew Judith well, perpetuated a conspiracy of calling her by her childhood name, “Eddie.”

 

I like this suggestion. I may use it if you don't mind.

 

No worries. It's why I posted it.