You have to fail two classes in order to repeat the whole year. If you fail only one, you'd still could pass (but had to go to additional schooling). But if you had failed two, then there was something wrong going on with with the whole year, and it had repeated.

Usually it was kids that had some discipline problems. Perhaps ADHD, etc... but at the time, no-one cared about those aspects as school psychologist were not a thing at all.

If you failed more than one year in a row, then the teachers will just give up on you and let you just slide the next year as long as you just showed up.