University website says "when such conduct [unauthorized collaboration] is common, it becomes acceptable" - does this mean plagiarism is allowed? I have been accused of plagiarism from my prof, since I have an answer from a Facebook page based on our course, and a lot of students got their answers from there. The Facebook page has previous labs and assignments from graduate students or senior students who have already done the course, thus there were around 14 students with the same mistake including me. But I went through my university web site and it states for the TEACHING ASSISTANT (TA) that:


  Academic Dishonesty in Laboratory Environments
  
  Academic dishonesty is a serious problem in undergraduate labs. This
  is partly because the culture of lab courses sometimes fosters
  plagiarism. Lab exercises may remain unchanged for years, making it
  relatively easy to obtain lab reports from previous students. Since
  students generally work in pairs, the distinction between acceptable
  and unacceptable collaboration can become blurred. And sometimes lab
  work is simply not taken as seriously as other scholarly work. The
  main forms of academic dishonesty in laboratory classes are:
  
  Plagiarism in laboratory assignments and reports
  
  Among some students there is an academic culture that accepts a
  certain degree of academic dishonesty in labs. Students buy and sell
  lab reports from the previous year and some try to "help" junior students
  by "handing down" lab material. When such conduct is common, it
  becomes acceptable, and many students may not realize the element of
  dishonesty involved. In addition, in most cases the entire class is
  writing up the same report, so there is bound to be an enormous
  exchange of information.


I think the paragraph above says that at this point this academic dishonesty becomes acceptable. Does that sort my issue at any point?
You are misreading that passage.

"It becomes acceptable" is intended to mean "acceptable to the students".  To rephrase:


  When such conduct is common, students come to believe that it is acceptable.


But such students would be mistaken in that belief, from the institution's point of view.  I think the clear meaning of the passage is that such conduct is not acceptable to the institution and is considered plagiarism and academic dishonesty.