Usually, the marking system is divided between participation in class, weekly exams, end of semester exams, and homework, which has to be both accurate and turned in on time.
The GPA system means your final grade in any class will be the average of all the components of the grade. If your homework is rarely handed in, you'll miss those marks and you'll probably fail the course. If it's late, your grade is automatically lowered. You will not get an A for work that's A standard if you hand it in late.
Before the semester exams, most teachers will send an email detailing the grade a student needs to aim for in order to maximise their course grade. A few weeks before that, failure warnings will have been sent home if the student is hovering on the brink of failing the class. For students who are in a sport, weekly grades are sent to the Athletics Office, which coordinates the athletic study halls for students who are temporarily suspended from the sport until their average rises.
You don't have to wait until the end of the semester to find out how you're doing in any given class.
Most high schools above a certain size have multiple levels of classes in core courses, with each class forming part of a track. Students are usually on a track where they will have to challenge themselves, but not at a level where they are completely in at the deep end. Education can be very individualised. Students from all four years can find themselves in the same classroom, depending on the stage of their particular track, especially in art-related courses, but also in language classes, mathematics, sciences, English, history..