S4 is the equivalent of Y11. People get confused by the 6 month difference in age cut-offs - and the fact that Scotland is more flexible and allows kids who are not yet 5 to defer to the following year (in practice, only Jan/Feb and possibly Nov/Dec birthdays).
Both systems have 13 years "formal" schooling, in both of which the last 2 are "optional". Reception is an "additional" year which is not compulsory.
Highers are roughly the equivalent of AS levels and Advanced Highers of A Levels, but they have different strengths.
I went to a predominantly English University - albeit in Scotland (St Andrews
) - straight from S5. I was at the "old" end of the year so was 17.5 when I started. So if I'd done S6, I'd have been 18.5.
Almost none of the English students jumped straight to 2nd Year, but the lecturers only nominally covered the difference between Higher and A Level, really rushing through it. It meant that 1st Year was tough - but I coped.
I can remember thinking the opposite of the OP's impression, based on my experience of my fellow English students: I used to wonder what they did with the two years of an A level course and only 3 (maybe 4) subjects, as with only 6 months study (allowing for exams/study leave/holidays) compared to 18 months, and a broader range of subjects, we managed to achieve 2/3 of the level of the A Level students. 

Ds is currently doing his Highers (sat his Prelims before Christmas) and I can assure you he had/has homework and is encouraged to revise on a daily basis (by his teachers, not by us!
). And the fact that he is doing 5 Highers (the norm in Scotland - although my school c40 years ago
allowed some of us to do 6) means he is doing a broader range than he would be doing in England.
Next year he is planning on doing 2 Advanced Highers and a crash Higher as well as voluntary work (possibly helping with coaching at the School of Rugby).
He is at the "younger" end of the year (technically in the middle but because of deferrals, on average he is one of the younger ones), so even after doing S6, he will not quite have turned 18 when he starts at Uni.
His school is one that allowed 8 Nat 5s - but those that don't allow so many tend to have covered a wider range of subjects to a higher level to the end of S3, theoretically allowing them to make a more informed choice in S4 and/or pick up other subjects as Crash Highers in S6.
He could have done all 3 Sciences for Nat 5 but chose not to (might do Biology as his Crash Higher in S6). Nevertheless, he is doing a broad based range of Highers: English, Geography, Maths, Physics and Chemistry.
Most years his school (a highly mixed demographic, inner city state school in Glasgow, with many languages spoken) has one or more former pupils going to medical or vet school. This is despite extra tutoring not being the norm. There are also regular Oxbridge admissions (although fewer than there used to be, as people are choosing to stay in Scotland due to the need to pay fees in England).
So the Scottish system can't be all bad. 