Since it was a communist country, everyone had the same conditions (crappy), and everyone had the same fricking clothes (only two clothing factories on the whole country), and same teachers (and no private schooling allowed), and we had the same books, and no calculators.

And it is clear, if all conditions are equal, still people are not born equal. Some kids are just smarter than others, and some are just dummies, or don't care about school. Since everyone was the same ethnicity, you can't blame 'systematic x'. It was all equal. A doctor and a nurse probably lived in the same apartment, and had similar wages. (think Cuba, or North Korea today. Albania was like the North Korea of Europe).

Grades/marks were 2 - 10, 2-4 was failing, 5 was passing, 10 was excellent (and hard to get).

Grades were given in the basis of

1. homework,

2. blackboard interrogation (you had to solve and explain everything in front of the class, similar to a whiteboard coding interview),

3. Exams, flash quizzes or pre-announced.

You had three types of students:

1. Great students, and are aiming for the 10s (and usually get a 9 or a 10).

2. People that struggle to do the basic, and just want to pass the class and are aiming for a 5 or a 6.

3. Everybody else that got a 7-8.

Every exam or homework, was done with this in mind. You usually had 3 type of questions:

1. Super basic, <- If you were in class, you could do it

2. Medium, <- Some thinking is required

3. Advanced <- Usually only the really good students got these

The teachers knew, that some students would struggle, and they will let them pass if they just did the basic effort. If a student failed even that, they would have to go to 'summer class' and take additional classes and exams to pass. If you failed a class, you failed the whole year and had to repeat every other class as well.

Also, the school over time divided the students by grades. Each cohort-year, might have 4-5 classes (of 30 students). Your class was the people that you studied with, and did everything. The top students usually were placed together, and spread in the class A, and B. The rest were put on the other classes.

So, the top classes had only excellent students, or average students, but not of the failing one. The idea was that failing students will just drag down all the top students and not let them excel. A top student can make an average student better, but there was little chance to do anything with a failing one. If you were initially a failing student but started doing better, you could move up to the better classes. This is similar to the soccer relegation techniques of most soccer leagues in Europe.
But: Every class, had the same subjects, and the same load. Even the failing students had to do pre-calc. But the teachers would just be much more lenient on them. Eg. do the very basic, and they will let you pass. But if you wanted a good grade, 8 or higher, you had to work your butt off.

This is totally politically incorrect in today's environment, but even communist Albania knew better. Some people are just not smart at all, and it is better to let them just do the bare minimum and pass, meanwhile let the smarter kids do more advanced work.