Let's make a deal: if you agree to follow the rule, I'll explain why it's not really a rule. Cool?
The difference between active and passive voice is emphasis; active voice emphasizes the subject; e.g., "JOHN answered Jen's question," while passive voice emphasizes the object; e.g., "JEN'S question was answered." It's simplistic to say that one is better than the other. Teachers insist on students avoiding the passive voice probably because some students (not you!) intentionally or unintentionally try to weasel out of exploring topics by using it. For example:
"The US Constitution was widely accepted."
Wow, what a boring platitude that doesn't really tell me anything--that's basically just a safe way to fill a word requirement. But if you force yourself to avoid the passive voice, then you might write:
"White landowning men accepted the new Constitution except in Rhode Island, which didn't ratify it until more than a year into George Washington's first term as President."
Well *now* you're saying something! (and more words too!)
(and hey, if you don't share that sentiment and want to argue that women, poor people, enslaved people, and native people also accepted it, go ahead...just be prepared to back it up)
So follow the rule and avoid the passive voice, not because the active voice is somehow better, but because it's good training to make sure you're always clear about what's being done and who's doing it. Like my mom always says, you can break all the rules you want, but you have to know what they are first.
And one last point. The reason I suspect that Barbara couldn't find a problem with your Lincoln sentence is that it's not in the passive voice. It's just a subject (AL's concern), the verb to be, and then an adjective. However, to Jacob's point, because it's for AP Lang, you'll want to keep the focus as much on rhetorical strategy as possible. So maybe, "Lincoln's powerfully emotional language make his concern for the country evident," or something like that.