How to deal with arbitrary grading at a European School?

We have been here for a couple of years now, and our two kids attend one of the European Schools.

Some of the grading seems quite arbitrary. So much depends on things other than written tests, and teachers seem to have a lot of leeway in deciding grades. It can feel like they can just decide between an A and a B based on their own judgment. There does not seem to be a clear or consistent system.

We are also concerned that if a teacher does not like a student or the parents for some reason, the child can be at a disadvantage. Other parents have advised us not to speak with the principal because they are worried about retaliation.

Our child is stressed and unhappy because of this. Has anyone dealt with something similar? What is the best way to handle it constructively without making things worse?