School Play - S3-E19
Other mistake: After Helga gets mad at Arnold in the beginning, as Helga says, "What an annoying little smarty pants know-it-all," the lunch lady's body is invisible through the glass on the counter, leaving only her upper half visible. (00:01:12)
Answer: The show is based in present day times. Plus most cars aren't old fashtioned, I recall seeing a limo and not to mention a cherry picker, etc.