A few general thoughts:
1) There's nothing wrong with a teacher providing additional classes for students in his classes, provided that (i) it's free, (ii) it's optional, and (iii) it's available for all students in the cohort at that school.
2) By "optional", it's important that no content is taught that is not taught in class; no official assessments are done; no penalties are applied to students who do not attend.
With respect to your situation: it will be important for you to distinguish between two possible explanations for the higher grades of the other students: (i) they perform better because of the extra learning they have done, (ii) a lack of objectivity from the teacher. You'll need to have evidence that it's not the former.
With all that said:
This makes my skin crawl, and if this accurate, I would be concerned not only about this teacher's partiality, but also his suitability to be working with teenagers.
My response to those:
1) His classes tick all those boxes, but I just choose not to attend them because I don't personally like his teaching style, so instead I have a physics tutor who I relate with.
2) From the word of my classmates, they have told me that they are done with unit 3 whereas we are still mid way unit 3 in class. Therefore, he sees those who attend his classes as the 'advanced' ones who have completed the whole unit before the others, however, he does not know that there are other students in the class (not only me) who do not attend his classes but are just as ahead, if not more. This saddens me as myself and a few others are spending a load of $$ to our tutors to learn the content, just to be seen as less than his chosen students.
I can almost assure you that it's his lack of objectivity because our first SAC required us to make a model of the same type (literally the same task for everyone), yet those who attend his classes (roughly 5 people) all achieved the top ranks respectively, i.e: rank 1, rank 2, rank 3, rank 4, rank 5. He even admitted that my work was very detailed but still put me as rank 6 (it's his first year teaching a 3/4, he has no physics background whatsoever + he doesn't know the effects of ranking on your study score). Just to confirm this, I asked a friend of mine who is rank 3 who attends his classes if we could compare our reports for the model after we submit it. There was a clear contrast between our work, of which I don't want to sound egotistical but mine was a lot more detailed (he even admitted lol).