I wasn't surprised to see it in my CCD class. They're sixth grades. However, I was surprised that it cropped up in the forum from a well-versed Catholic adult!
So I figured I'd share my answer here since it's a more prevalent question than I'd realized.
We technically have the Romans to thank for the title of "Good Friday."
See, back when the St. Paul started preaching the "good news" of Jesus Christ, there was another word you might be familiar with in constant use... "gospel."
Before us Christians usurped it as our own, the word "gospel" had a very specific connotation. Since Rome enjoyed conquering every community known to man, they were frequently in far-off places fighting a variety of different people. As a result, they needed fast couriers to let the various generals (and Caesar) know if they were winning or needed backup.
When these couriers skitted back to the capital with news of a victory, they called it the "euangelion" (which is actually the Greek word for evangelization or "bringing good news"). The good news was victory for the people. Oddly enough, it also referenced the official laws and privileges that these new Roman citizens could be assured of if they played nice and followed Roman authority. That, in turn, was the actual "gospel."
So apply this knowledge to what our Christian gospel actually is. St. Paul describes it best as the death and resurrection of Jesus. From the Throne of the Cross, Christ defeated the enemy and assured salvation for those who would accept His Authority. It makes perfect sense, then, why we would consider that first Good Friday to be "Good." It was the true trumpet of humanity's "gospel." That act secured for us victory in addition to the privileges that come with being a child of God.
As the years went on, this word was picked up and converted into "Godspel." It was a Germanic combination of "God / good" and "story / message." That's why most of us today understand the word to mean "Good News." Originally, however, it meant an entire group of people were welcomed into the fold with privileges and rewards so long as they agreed to abide by the authority of the one who conquered their territory.
In other words, Jesus came to earth, conquered it through His Passion, Death and Resurrection, and gained for us the inheritance of eternal life so long as we submit to His Will (which is nothing more than loving one another as He, Himself, has loved).