I'll try to explain to the best of my ability. Basically, you are more likely to flunk out before your 4th year.

You can't just go take Step 1 and Step 2 CK without passing an NBME comprehensive (basic/clinical) science exam. There isn't a universal passing score; its up to the dean to set that score. One school may require a 72 (=205) and then another might require a 67 (=192). They'll give you 3 attempts to pass, and if you don't score EXACTLY a 72, you're dismissed. Also throughout the years, they continue to increase the minimum. A few years ago you could take step 1 after scoring a minimum of 60, 64.

The latest is they can track how much time you spend on these mandatory USMLE review courses. I'm currently doing a kaplan course online. I was sick for a couple days so I didn't logon. I recieved a warning email from my dean + some Kaplan advisor.