At the beginner (high school / undergrad) level all Lisps are more or less the same. The finer points of Lisp-1 vs 2, CLOS, etc. are not comprehensible without

At the beginner (high school / undergrad) level all Lisps are more or less the same. The finer points of Lisp-1 vs 2, CLOS, etc. are not comprehensible without a lot of prior work.. and by that point you should optimize for real world practice anyway IMO (something which many university courses do not do). If we're talking middle school then there are some arguments to be made that Racket's easy-to-use graphics libraries are more conductive to learning. It will never be Flash though.