What makes a journal scientific?
There is no general definition for scientific journal. The Wikipedia definition is as vague as reality is. This is because there is no clear definition of scientific. It is a case-by-case distinction - not only for journals, but also for individual articles in a journal or one method described in an article. Additionally, scientific and predatory are not mutually exclusive. 

Your instruction to your students to only use scientific literature is the key to the answer to your question: What is scientific and what is not depends on the concrete content of an article or the applied method. To distinguish between these two is the first step towards scientific thinking.

An answer on a list of predatory publishers can be found in this question here.