Are the gospel authors who we believe them to be? What I'm wondering is whether the actual authors of the book of John, Luke, Matthew, and Mark are who we believe them to be. I go to a Catholic high school, and in our version of the Bible it says, in the author section for each gospel, something like "traditionally the apostle John", so does that mean it could have been someone else, and we just say that the author was John the Apostle when it wasn't?
I will start a little off-topic as it is relevant to do so.  

An answer to this question would not have been easy if there was no early persecution of Christianity. It would have been a lot easier for detractors of Christianity to make a point that all the writings that we have today on which the foundation of Christianity is based upon, were in fact all cooked up and doctored with the help of the political and reigning powers at that time. But the fact is, all the worldly forces were trying to wipe out nascent Christianity by persecution and destroying all their literature. All that we have today has passed through a most turbulent period and has survived under most cruel persecution.  By the time Christians could live a serene living under a favourable reigning emperor, all present day Gospels were already in use , as independent books before they were canonised in this favourable time. 

To answer the question:  We don’t have the original documents in our possession of any of the Gospels, thanks to this persecution.  Agnostics raise this point in casting doubt on the authorship of Bible books. But seen from the table below it can be said of any other ancient literature on the surface of this earth  and that too with no attempts of supression. That is the case with every historical writings and we find that evidence for authenticity of Gospel writers is much stronger than any of the other historical books.  

     

Apart from this shortest timeline for the earliest copy, there are other numerous external written sources, which are but only positive indicators (and absolutely nothing negative) in early history, that the gospels were written by the persons to whom they were attributed by the early church (Matthew, Mark, Luke and John).   

A point to note: The two of the gospels are ascribed to such minor characters as Mark and Luke -- neither of whom, by any accounts, were themselves neither disciples nor the eyewitnesses is solid indirect clue they were who they were. If early Christian evangelist wanted to acquire credibility for what they were writing (or forging as some claim) and preaching they would undoubtedly have attributed it to someone like Peter, Thomas or James. And precisely this was what the later second and third century Gnostic gospels did. At least Christians are saying that all the Gospels were written under the guidance of Holy Spirit and not claiming that they were handed down by God Himself or were found buried somewhere in the ground.  Or to think extremely they did not concise all the four gospels in to one book and say that it was written or dictated by Jesus himself.   

This link cites the compelling evidence  where it is explained in detail.   

Additionally further information for Gospel of Mark is found in this link and Similar readings can be found here for Gospel of Luke and another link for Gospel of Mathew