Because of the difference in cut-off dates, sometimes Scottish schools will accept the number of years schooling rather than pure age - have you talked to the school/Education Department yet? We are more flexible regarding age (but bizarrely, once you've started, less flexible).
S1 is not the equivalent of Y8 - it is the equivalent of Y7. (The difference starts at P1 which is the equivalent of Reception; we also have different cut-off dates, between March and February - plus the ability to truly defer the child turns 5 after August in practice only January, February and maybe December kids might defer).
So ds, who was 12 in September (born 2000) is in S1 and is in the young half of the year, despite technically being slap bang in the middle, as some of the "youngest" kids were held back and only started in the same year as him (one of his friends in the same year will turn 13 in December - but another one, whose parents chose not to defer him, will turn 12 in February).
The S2 (Year 8) child will be doing Curriculum for Excellence, which is still sorting itself out. Don't know the detail but most schools still seem to be choosing their subjects at the end of S2.
The S4 (Year 9) child will be doing (the last year of) Standard grades I think. These are broadly equivalent to GCSEs.
The Year 11 (S5), midway through A levels kid could probably sit Highers - but we tend to sit more - so you'd probably find the AS Levels similar. We tend to sit most Highers in S5 and then maybe do a few more in S6 or some Advanced Highers (or even 
A Levels). That's what the universities prefer anyway. It means a lot of work in S5 - but it is possible to go to Uni at the end of S5.
I got a conditional acceptance from an English Uni (Leeds) for my Higher results - but chose not to go there (went to St Andrews instead - at the end of S5) - but that was many years ago. One of my school friends went to Cambridge to study medicine and the others went to Glasgow to do medicine - they all stayed on to do S6 but were given unconditional acceptances on the basis of their Higher results.
Re the Latin - it depends on the school, as Latin teacher are getting in short supply. Ds' school taught Latin until last year and I'd have made encouraged him to take it but unfortunately the teacher retired and they've not been able to find a replacement. Instead he is learning a 2nd MFL (Spanish this year - if he'd started next year it would have been German as they alternate years) over and above the French he had already started at primary school.