Since it's for high school, and is just a unit, I'll assume it's meant to be a general overview of Christian history, and not the Bible or theology?

I wouldn't go too deep into the Bible, if I were you. It's obviously important, but try to keep it very general. I'd say in the first day (assuming class is around 50-70 minutes long - that's how long my HS classes were, at least), explain the Jewish component of Christianity, and how they're connected (the NT writers were all Jewish, the Messianic Prophecies leading to Jesus, etc.), spend most of the time explaining first century Christianity, oppression under the Romans, martyrdom, and go up until Constantine legalized Christianity in 313, the canonization of the Bible.

How many weeks do you have? I'll assume not many. Keep it brief. The important things to discuss to get a general feel for the very broad history of Christianity are the rise of the Catholic Church, the Protestant Reformation, and then the splintering into different global traditions.

For the RCC: Move fast, there's a lot to cover here. I'd suggest skipping over some of the early medieval stuff, since things really start moving later on in the medieval period. Explain the connection between the RCC and political institutions, and Catholic developments, spiritually (sacraments, confession, icons, sainthood, etc.). Delve into the corruption, buying of church offices and indulgences work well here, and also create a lovely segue into...

The Reformation! I'd spend a full day or two on this, as it's super important. Martin Luther, 95 Theses, and the Diet of Worms. Include other Reformers, like Calvin - especially if you're going to discuss American Christianity at some point. The Anabaptists are important as well, if you want to talk about America, as they're (kind of obviously, form the name) who the Baptists come from, and more importantly, the Puritans who travelled to settle in America (AKA, the folks behind the Salem witch trials).

After the Reformation, talk about the Renaissance and the Inquisition. Obviously these are dark times for the Catholic Church. Monasticism did some interesting things around this time period though, if you're interested in discussing that (look up Teresa of Avila and Catherine of Sienna). Also, the Jesuits were created to be missionaries around the world, they went to China, India, South America, and North America, and had very experimental and controversial methods (both good and bad - see India and China for good, see South America for bad).

Onto more modern stuff. American Christianity is really interesting, and I'm going to go out on a limb and assume you're American? It seems like most of reddit is (I'm not, so I'm sorry if you aren't either, from a fellow oft-assumed-American). In the eighteenth century, the Protestants in the US became obsessed with revitalizing Christianity, there was a lot of hellfire preaching (look up "Sinners in the Hands of an Angry God") and this is also where the tradition of biblical inerrancy comes from (thinking that the Bible is 100% factual, literal truth). This belief caused a lot of fracturing in American Protestantism. Things got really interesting with Darwinism and the Enlightenment. The twentieth century wasn't much better (look up the Scopes trial, it's pretty famous).

There's so much to discuss with Christian history, as it goes pretty much hand in hand with Western/European history. There's a ton that can't be covered, even in full-year university level courses. I can't give a huge list of sources now, but if you want to PM me later I can dig some up for you. A really great survey source is Justo L. Gonazlez The History of Christianity, there are 2 volumes. He's used in a lot of undergrad level Christianity courses (including my first year studies). Very easy, interesting read!

do you have to cover all of history? i would honestly focus on the variability of early christian doctrine and non european christianities in places like india, nubia, and armenia. expand how they think of christianity.

Maybe we can focus down the problem so everyone could help you better. What exactly do you want the students to get out of the unit?

• Do you want them to know the history of Christianity and how its doctrine has changed over time? There's plenty of information regarding Christian doctrine and that tends to be a defining factor in the constant subdivision of denominations (e.g. Protestantism vs Catholicism)

• Do you want them to understand the doctrine of Christianity and compare/contrast it to other religions around its inception? Related, how much focus do you want to spend with its growth over time? The growth of Christianity and how it has stuck in Europe for some time is quite interesting, at least in my opinion.

• How would this unit relate to other units for this class? Hopefully it will be thematically consistent with other religions for this course. Perhaps you should look at that first.

• How much freedom will you have in making this unit? Is it more of a lecture/seminar or more of a discussion with the students?

I think if you could be more specific in what exactly you want your students to get out of the unit, the better off you'll be able to focus your efforts to making it a better unit. So I ask again, what do you want your students to get out of the unit? If you can answer that, I'm sure everything else will follow.