Jacob learned what people started to farm at school.
The principle of clarity requires that the meaning is clear not only is to you (the writer), but that it would be clear to the average reader. Echoing the previous comment, it's unclear here whether "what" modifies "people" (i.e., "which people was it who started to farm"), which is not what you meant. Further, the prepositional phrase "at school" is placed too far away from the action it describes, "learned". So, moving it yields "Jacob learned at school [about] what people started to farm." I might even add, for clarity, "what kinds of crops": "Jacob learned at school what kinds of crops people started to farm." Further, I'd want to know why they only "started" to farm. Didn't they actually farm? Is the sense that farming was new to them, or they had returned to it after some hiatus? (The answer to this question depends on the context.)