"I took a day off from school yesterday."
I really wish it would mix in a whole lot more plain language, or even start with that and mix in polite later. It's weird when there's just one sentence like this, but knowing the plain form of verbs is very important.
The main way that more interesting sentences are built up in Japanese is that subclauses (basically any sentence) ending in a verb or い-adjective can behave like an adjective and be attached to a noun. But the ます/です form can't really go there, so until you learn plain verbs, it's really hard to express a lot of ideas.
Well, almost. You have the right idea, but the formal version of this sentence would end with やすみました rather than just やすんだ