I was reading from the script. I haven't noticed any scenes at Perry's Pizza yet, only Swenson's Ice Cream, Carl's Junior, Mr. Hand's History Class and so on. Checking the film locations, there is no mention of Swenson's, only the All-American Burger place and Perry's Pizza. My guess is that the script called for Swenson's Ice Cream but that part of the script was filmed on location at Perry's Pizza, Garden Grove, California. Jennifer Jason Leigh acted the role of Stacy Hamilton.
imsdb.com/scripts/Fast-Times-at-Ridgemont-High
movie-locations.com/movies/f/fasttimes
eatinoc.com/oc-food-reviews/perrys-pizza-garden-grove