This is a great question and it is nice to know that high school students are actually studying Justinian and reading Procopius. With that said a good general rule of thumb is to know that all authors are biased. For example all authors have to be selective and emphasize certain things at the expense of others. Hence, the question is not whether authors are biased. The real question is what their biases are. This is where the work comes in.
When we look at the works of Procopius, especially the Secret Histories, we see an author that is completely disillusioned with Justinianand Theodora. The work focuses on salacious details and makes every effort to show the evils of the royal couple. For example, Theodora would love to perform on stage nude and miraculously Justinian's head would disappear. These details are added to show his hatred for Justinian and Theodora. In light of this, Procopius is seriously biased.