Scheme is one of the languages used in Stuy's intro CS course. Like any other language, there are pros and cons of using it to introduce students to CS. Here are some of the reasons why I like Scheme as a first language and why despite that, I don't recommend it in many cases.

Scheme certainly isn't mainstream. That's not the main reason why I like it but it is a side benefit. It's less likely kids come into class knowing it which creates a level playing field. The few times I've had a student with a background in Scheme, they've been good, helpful kids rather than ones that tried to show off and dominate the class.

I like scheme because it's simple. It looks different than other languages but is (+ 3 4) really that different than add(3,4) or 3+4?

We can tell kids "everything's a function." While technically not true, and I tell my students that it isn't, it's close enough for our purposes. Everything's a function and the way we write a function is:

(f a b ...)

Apply function f on arguments b c …. So we have (+ 2 3) to add 2 and 3, (and a b) for logical and (= 3 4) to test to see if 3 is equal to 4, etc. We also have the same basic notation for things like conditionals. In java, we might write:

In scheme, it would be:

(if (> a 5)
5
10)

Even though we're fudging the rules, we can lay out a handful of rules for the kids such as lambda and define follow a similar form to create and name functions:

(define add2 (lambda (a b) (+ a b)))

where (lambda (a b) (+ a b)) returns a function that adds a and b and define associates it with the identifier add2. Now we can use the function: (add2 5 10) which evaluates to 15.

With lambda, define, if, and a few more basics, it's off to the races. The simple, consistent syntax makes things easy for the kids and the constrained tool set lets us focus their way of thinking.

Of course this wouldn't work had DrScheme (now Racket) not existed. That gave us a nice learning IDE to work in.

Using Scheme, we can not only cover the basics like conditionals, abstraction and such but also deeper concepts like recursion, map, filter, and reduce. When we move on from Scheme, the kids also have a frame of reference for all the other languages and paradigms they use.

There are downsides, however. Outside help is harder to come by and there is some pushback from parents and it isn't as pretty and flashy as some other tool-sets. Scheme worked at Stuy for a number of reasons. When I created the class that used it, there were no good pre-AP classes in the school and I had a pretty good reputation as a teacher. The class also used multiple tools with NetLogo being the primary other one and tackled a variety of CS and non CS topics. First time through, reputation and buzz filled the classes and after that, the class developed a solid enough reputation to fill itself.

It also didn't hurt that our program developed from the inside out. First, we had APCS-AB on the books as a junior year course, then I added our post AP senior year electives. Since it was easy to show what the kids were already graduating with, it was easy to diffuse any pushback from not using mainstream tools in the first class.

Scheme worked at Stuy and I feel that it still does. Would I use Scheme for a first experience in a drop in program at an after-school where kids might not come back if they don't get to a payoff immediately? Probably not. Would I recommend it at a school where I want to integrate CS with some of the other subjects quickly, again, probably not (except for integration with algebra where Bootstrapworld is a terrific program).

In any event, if you're a high school CS teacher and haven't played with scheme or some other functional language, I highly recommend it - you might not actually ever teach it or use it but it can really help develop you as a programmer and an educator.