[plt-scheme] Sandboxed evaluator for Beginner Scheme

I am trying to automatically evaluate code under BSL. The Scheme Sandbox has
support for specifying BSL as the language, but somehow it's not behaving as
I would expect.


Welcome to DrScheme, version 4.2 [3m].
Language: Module.
> (require scheme/sandbox)
> (define e
    (parameterize ([sandbox-output 'string])
      (make-evaluator '(special beginner) "7 (check-expect (id 1) 1) (define
(id x) x)")))
7
> (get-output e)
""
>


I would expect the 7 to appear in the string that results from GET-OUTPUT,
followed by a newline, then "The test passed!"

I am using calling MAKE-EVALUATOR wrong? I should be using some other
library, such as lang/run-teaching-program ?
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