Recently, I've been reading a bit of 鲁迅 and I've run into the word 凡事 a number of times. Everytime, I see it translated it seems to mean 所有。 It seems to me that 凡事 is either an older usage or is much more formal - is that correct? As well, are there any ways in which they are used differently Ex. Could I say "凡事（的）学生都很乖“？
1) "凡事" means "所有的事". So it is "凡" that means "所有", rather than "凡事" itself.
2) In effect, "凡" says more than "所有", but this distinction is boring and insignificant. I would say this: Logically, "所有的學生都很乖" is equivalent to "凡學生都很乖".
3) "凡" has way more meanings and usages than "所有".
You might not want to use "凡" to replace "所有" in spoken Chinese, for doing so may very well cause confusion or even give people an impression that you are trying to be cool...
First, a little background: Most of 鲁迅's writings were done in the middle of 白话文运动--to write with plain Chinese instead of arcane literary Chinese. Unfortunately, 鲁迅's way of using plain Chinese was not as influential as chairman Mao's, most of which actually have Huananese origin. From today's point of view, 鲁迅's "plain Chinese" is rather idiosyncratic.
凡事: Under most/all circumstances.
凡事，指不论什么事；所有的事。语出前蜀 魏承班 《谒金门》词之二：“愁倚画屏凡事懒，泪沾金缕线。”
所有: 整个;全部；一切。有时也指一定范围内的。All, all within a scope.
"凡" means "usual" with a sense of "in most cases", etc. It also means "every", especially when supplemented with "必", "都", "皆" etc. We need to distinguish them by the context of the sentence.
For example, in supermarket, it says "凡購物滿一百元可獲九折". Also, during a wedding, the priest often tell the couple to "凡事忍耐".
Obviously, "凡" carries different meanings in the two scenarios.