When talking about the future in English, do whatever you want. For almost all practical purposes, there is no real difference in meaning between the available forms. However, it is better to place "on Monday" at the end of the sentence. All of these are essentially identical: You will have a test on Monday. You are going to have a test on Monday. You have a test on Monday. "You are having a test on Monday" has a slightly different connotation. It implies that the students already know it, so it's a reminder rather than an announcement. "Remember, you are having a test on Monday, so be sure to get plenty of sleep."

All three are grammatically correct ways of talking about the future, as is "you will have a test on Monday". As long as you use a time marker "on Monday" it's understood that you're talking about the future.

They are both grammatically correct. If you say 'On Monday, you have a test", it means that on Monday, you are going to have a test. If you say 'On Monday, you are going to have a test," this conveys the same meaning.