I try to find the “least disruptive” stims. For me that has been a ring on my index finger that I can use as a fidget - most people don’t even notice. When I can’t use my ring , I count something silently - a tally mark each time a teacher says a specific word or how many people in the room are wearing a color or things like that. Another silent one that works for me is picking a category and going through the alphabet listing thinking of something for each letter in the category (example: category - fruit, apple, banana, cantaloupe…). When I have to be vocal, but don’t want to make noise I either whisper (if possible) or just mouth the words/sounds.
I hope you find something that works for you.