How to degender "separate the men from the boys"?

In addition to Jacinto's answer, you can also avoid the human element altogether and use a phrase like:

Math teacher: "The ability to do proofs of this type is what separates the wheat from the chaff in this class."

The Free Dictionary describes the phrase as meaning:

to separate what is useful or valuable from what is worthless; to choose what is of high quality over what is of lower quality

In response to the OP edits:

You're going to have a hard time finding an answer that doesn't slight those that don't stack up (at least on some level) because your original phrase "separating men from boys" places a measure of disrespect on the "boys", essentially calling them weak, immature, and less macho.

Any equivalent phrase, no matter how degendered, will place some level of inferiority on the, ahem, inferior.

At some point, you have to step back and look at what you're trying to say. If you're trying to say that a certain process weeds out inferior special education evaluators, the men/boys or wheat/chaff expressions fit nicely. If you're trying to encourage a student, letting them know that this task will be difficult without placing inferiority on those that don't catch on, you probably shouldn't be making a "separates this from that" comparison.

I'd instead go for something that focuses more on the victors rising to meet the challenge or the challenge itself. These immediately come to mind:

Math teacher: "Learning proofs of this type is a real trial by fire."

or

Math teacher: "The ability to do proofs of this type is what determines the cream of the crop."

or

Math teacher: "The crème de la crème in this class will be able to do proofs of this type.

or

Math teacher: "For some of you, learning this type of proof will be your crucible."

I like the pair grown ups/kids or grown ups/children. Here’s an example from Oxford Learner’s Dictionary (go to “noun” and click on “More examples sentences”; emphasis mine):

‘In a festival where easy laughs are mostly the order of the day, this is a serious piece of work that separates the grown-ups from the kids.’

And another example from The Business Insider (2014):

Compromising is a humbling lesson that separates grown-ups from kids.

I’d say this simply removes the gender while keeping the meaning and informal tone of the original men/boys. But if you want to focus attention on high achievement, without equating potential low achievers with anything in particular you could go for:

By being able to do proofs of this type is how you rise above the average.

Training and experience with the specific disability is what it will take to rise above the rest.

You could drop the particular idiom of "separating A from B" and wind up very close with testing their mettle:

The ability to do proofs of this type is how we test the mettle of students in this class.

TheFreeDictionary.com defines mettle as

The ability to meet a challenge or persevere under demanding circumstances; determination or resolve

...which I feel captures most of the nuances of separating men from boys.