In my university there is an issue regarding the step from high school to college. There is an entrance/admission exam with high competition in between the two. Nevertherless, the fail on disciplines rate is quite high, sometimes incredibly high.
Calculus: calculus continues from where high school has ended. To learn limits, derivatives and integrals you must be very well versed in functions and real number operations. Even though there is an admission exam, sometimes 80% or more of a class fails. In my first semester there was a teacher (Calculus I) who teached like this: in every topic he would go straight into problem solving, without explaining theory behind all calculations being done. He would solve an example problem, then do another one, but this time stopping in the middle, facing the class and asking "what's the next step?", then some ppl would randomly guess "factor! expand! substitute x by a!". Now, in the second semester there is another teacher (Calculus II), but this one draws graphs and explains theory. He did a diagnose test to see how much ppl learned in the first semester. Surprisingly, the half of the class who didn't fail and did pass, were unable to properly answer questions such as "explain why the function is continuous" or calculate some exponential function limit.
In a 0 - 10 scale. =>5 means approved, <5 means fail. About 50% passed, 50% failed. But if you count ppl who left, quitted before the end of the semester, fail rate would be over 50%. Majority of the ppl who passed, were in between 5 and 6 grade score grade.
Tip: don't *kill* youserlf with abstract proofs from the start. It's better to first solve many exercises, then go back to that proof. Some of the proofs require knowledge about things that are seen at a much later stage, it's kinda impossible to understand the proof if you skip the easy/medium exercises.