filtered

When a subject and/or class, usually in higher education scientific fields, is so convoluted and hard to follow, that it is impossible for one to succeed in it. This might be due to the professor, or the subject itself. In this case, the subject acts as a filter, preventing you from moving on to more advanced subjects that built upon it. This can often extend or apply to specific concepts included or related to a subject or class. In any of the above cases, it is fair to say that one has been filtered.