[plt-scheme] Question on the teachpack world.ss

Can you explain me why this code stops, prints "End" at the toplevel,  
but not the text "Over !" in the canvas ?
Everything else fine.
Thanks (DrScheme v371, MacOS-X 10.5) :

(define (next world)
   (if (crashed? world)             ; ok
     (begin (place-image (text "Over !" 48 "blue") 100 100 (empty- 
scene WIDTH HEIGHT)) ; nothing happens !
            (end-of-time "End"))    ; ok
     (new-world ...)))              ; ok

(on-tick-event next)

By the way, it is a bit annoying to be limited to "advanced student"  
to run this teachpack, why not full Scheme ?

Thanks,

    Jean-Paul Roy