server.rkt

#lang racket
(require json
         web-server/servlet
         web-server/servlet-dispatch
         web-server/web-server
         web-server/dispatch)


(define-values (start mk-url)
  (dispatch-rules
   [("") index]
   [("json") json]))

(define (index req)
  (response/xexpr
   `(html
     (body
      (h1 "Hello")))))

(define (json req)
  (json-response #hash((studentId . 400))))

(define (json-response json)
  (response 
   200 #"OK"
   (current-seconds)
   #"application/json"
   empty
   (λ (port)
     (write-json json port))))
  
(serve #:dispatch (dispatch/servlet start)
       #:port 8080)

Yeah sometimes it's not easy to find the right tutorial with Racket because they're so excited that they can do everything. This is probably a better tutorial: http://docs.racket-lang.org/continue/