It'This semester is almost half of this semestver, so classes that are held in the first half quarter are going to do thebeginning their final exams.

I think it is hard for me to know what you mean here, because I am not familiar with your country's semester system.

Here is what my correction means, if it is incorrect, please leave a message and I will write a better correction:

This semester is 50% complete. This means the year as a whole is 25% complete, because there is still 50% of the semester left, and then another semester. So classes which are held in that first 25% are starting to have final exams.

Hopefully that makes sense.

The rest of the corrections are to sound a bit more natural, and for grammatical correctness. "going to do the final exams" is not what people tend to say about classes. A human can "go do an exam", but the classes do not answer questions in a test, so they do not "do" the exam.

I have to study for them, and additionally, some of theis semester's classes have a presentation or a midterm assignment, which are heavier workloads than usual.

People do say: "I'm working heavily at the moment" or " This is a heavier workload", however it sounds a bit weird by itself, so I added a word to make it clear what exactly is heavier than normal.