Maybe a random integer in range from 0 to 2 (if there are 3 options) and then do a branch that asks if it was a 0? If false do another branch that checks if it was a 1 and so on.
If 1 = true set class detective for example and do a boolean variable “class taken”.
And the next time it runs and it is a 1 again do a branch before setting class where you plug in “class taken” and if it is true, then run the random integer again.