Did most students who fail/get drop from the college know it? - eg. they find themselves struggling with the theory/sim, or do most of them think they are doing fairly well, and then suddenly they get told by the instructors that they have to pack their bags?
It won't be sudden, you get regular reviews in the sim and you generally get the tap on the shoulder when you've been 'at risk' several times in a row. You'll know if you're not making the grade and there are usually options, like recoursing or extra sim runs. It really depends on the individual.
In my case, I chose to leave because I was struggling and knew it was just going to get harder. I got through the theory okay, with lots of studying in the evenings and weekends. Once I got to the sims I just couldn't keep up with the pace. I was forgetting phraseology, missing possible conflicts, you name it, it all added up to me getting further and further behind in each sim run. Once I started going to work with a knot in my stomach, I knew it was time to go.