Most other countries DO have the repeat system, OP.
In terms of compulsory school ages, some places actually have compulsory school levels to complete. In my country you must achieve the equivalent exam to GCSEs - usually done at age 16. However it’s not unusual for 17 or 18 year olds to sit them for the first time at that age after having repeated one or two years.
There is no option to leave, say, with only grade 8 (normal age 14) if you’ve turned 16 and decided to stop going to school, because it’s compulsory to achieve the equivalent of that exam before you can enter the workplace.
For students who have genuine difficulties achieving that exam, and would be at risk of repeating over and over, there are “practical schools”, a bit like colleges or apprenticeships, where they can take more tailored courses that would give them an equivalent qualification and direct them towards a job, but it’s rare that children are allowed to fully leave the educational system just because they were held back.
Nobody bats an eyelid at an 18 year old sitting GCSEs with their 16 yo classmates.
Edit to add that if you were to leave education altogether before achieving the minimum required level / exams, you’d have lots of trouble claiming benefits or entering programmes designed to get young people into the workplace. It’s not a get out of school card - you must complete your secondary level in order to either progress towards uni exams, a more apprentice-like schooling system, start working or claim job-seekers allowance.