You're off to a good start. There are just a couple of things. I know you said that this sentence was in a context of students not knowing the date for the end of the Pacific War. However, just looking at this one sentence outside of that context, I would construe it rather differently.
The core of the sentence is
There are more and more people who are uninformed.
Here "more and more people" is my way of handling "人が増えている".
The speaker wants us to know that this is their opinion, so they add
I think it is the case that ...
Just putting these two parts together, we get
I think it's the case that there are more and more people who are uninformed.
So, we could ask the question, "why does the speaker have this opinion?" Well, they told us already, that's the part of the sentence that precedes ため. Namely,
Here is where is seems you got a bit more stuck about what's happening in the sentence. This portion of the sentence can be broken down into four parts.
[Something happens] a lot.
What happens a lot?
[something] unfortunately comes to an end
I'm using "unfortunately" here to get across the idea communicated by てしまう. I only know to do that because of the larger context of the meaning.
The something here is
Middle school and high school history classes
Putting these three things together gives us
Middle school and high school history classes unfortunately frequently come to an end
Notice that I use "frequently" to get across the idea of ことが多い.
Now the final part, why is this a concern? The context you were talking about was regarding students not knowing certain dates. So, the final piece of the puzzle is
before they teach about the Pacific War
So, putting this forth piece together with the three previous pieces, we get
Middle school and high school history classes unforunately frequently come to an end before they teach about the Pacific War
Now putting it all together, you get
I think that there are more and more people who are uninformed [about the Pacific War] because middle school and high school history classes unforunately frequently come to an end before the Pacific War gets taught**.
Technically, there's no passive construction in the sentence, but I'm just trying to render the last part a bit more smoothly into English.