It is rarely used in everyday language, at least in U.S. English. Dictionary.com gives these synonyms, which you can use against your teacher: evil, base, unjust, wrong. https://www.dictionary.com/browse/iniquitous

The word iniquitous describes a grievous violation of rights. It is usually reserved for crimes or actions that are wicked or vicious in nature. It was often used to describe places like the opium dens or houses of prostitution. If you want a word to describe extremely hard tests given by your teacher, you could use unwarranted, unjustifiable or unreasonable. The word iniquitous might be a good word for the actions of a teacher who beat or abused his students if they made mistakes on a test. Hope this helps

Iniquitous would mean that it is unfair, and therefore wrong on a moral level. An iniquitous system for instance would be a system that gives a certain individual or group an extreme advantage over others. The example you gave wouldn't work, because it would imply that your class was getting this exercise in order to disadvantage you, which I doubt. Even if that was the case I would refrain from using this word, because it implies a systemic unfairness, rather than a single exercise, or even a single teacher being unfair. I hope this cleared it up.