In addition to most of the steps outlined above by others I have a few extra/different steps I use.
Besides arriving early on graduation day once the crowd starts to appear I pause the game and very carefully use the resetsim code on the npc's to thin out the crowd {Do Not reset your sim}. This works nicely for me as I'm able to invite family & friends and my sim will still graduate with the extra trait.
Lastly I don't let my sim hang out and chat after I 1- send them straight back to the home lot since they have to be there to leave for home, 2- save right after they get their diploma and trait just in case of glitches or crashes after graduation.
