I am doing my PhD at a research institute, where we have just finished the draft of a 8-hands long paper to be published as a short book with a major editor. I have done the bulk of the work, i.e. at least 80% of the total. However, my coordinator sent me the draft back with the authors reordered by seniority, and I now figure as the last author although my contribution was the largest. I should mention this would be my first publication. Shall I react?
Anyway, you should discuss this with your coordinator and ask him why he prefers this order. My colleague is working in a similar area than yours and he is usually the first author, but there might be other reasons in your situation.
Even if you are not able (or not willing to) insist on your preferred order, there are other options to emphasize your contribution. For example you could add an "Authors Contribution" section where you can explain in detail who did which part of the work. Footnotes at the author names are a common alternative (e.g. "* These authors contributed equally as main authors" or "¹ AB developed the model ² CD wrote the manuscript").
This is not only a question of fairness, but there might also be a formal requirement for your PhD or later research positions like "You have to publish two papers as main author." If that is the case, you should ask your supervisor how to address this issue.
As a junior partner who is yet to earn a PhD, the ethics of it are of less concern to you than that you successfully complete your degree and find suitable employment. Yes, you are probably being abused here, but getting powerful people mad at you will have negative consequences - all on you.
I hate to see these situations develop and would (as a more senior person) have harsh words to say to your supervisor, but I can only do that as they have no power over me.
Hopefully this is only your first, not your only, publication. Hopefully you won't have people like this in your future. But you have to take a somewhat cautious route to that future.
It may even be, depending on the reputations of the others, that you will benefit from a visible association with them.
In other answers on this site I've suggested that ethical violations be reported to authorities, but anonymously. That doesn't seem to be one of your options here, I think.