Difficulty, or the perception of difficulty, is all relative, based on the your level of competence in assumed knowledge, how you learn bs how they teach, etc etc
How much more difficult was year 12 maths to year 11 maths?
How much more difficult was year 5 maths to year 4 maths?
It is all a natural progression assuming what you had done up to the previous year.
IMHO, the main thing that impacts on a student's performance as they move from school to Uni is the different teaching and learning approach required. The fact that it requires much more independence and initiative from the student, and they are often influenced by outside issues (turning 18, getting a license, going to pubs etc) can influence.
Also, the marking at Uni is often different. At high school, you get a mark of 7/10 and one often asks the teacher "where did I lose my 3 marks". At Uni, 7/10 is a credit/distinction. You might have got the answer numerically correct, but what is so special about that – anyone should do that if the lecturer gives you the equation. You need to "earn" marks by showing a level of interpretation and critique and understanding of the parameters in your response. Google something called Blooms Taxonomy.
