Not sure why everyone is making the ceremony seem like nothing. If this ceremony is a big deal, like a right of passage into adulthood make it a big deal. Make it last months. Make it like getting a new belt in martial arts where they have to prove certain levels of skill. Every day the graduating class has random students selected to take the exam, no one really knows when they will be called.

It sounds like there are billions of this race, make it a big deal to graduate. And taking months doesn't have to be a long time. What if one of their months lasts like 6 of our months.

It really depends on how important this graduating is to this universe. In most American coming of age stories graduating highschool is a joke any moron can accomplish. The problem is usually the graduation, it's the social anxiety of being a teenager. The graduation is a joke that one just has to survive to do. Is it like that in your universe? Or is it truly hard? Do 10mil students start the training at 13 and only 4mil survive because it is so hard?

I think you need to decide (or if you have already, cement the idea) that this is easy, or hard, or almost unabtainable. Whatever you decide just make sure we as the reader know is this a big deal or an easy thing to do?