"classes with less than or equal to 100 students" -- seems awkward "classes with the number of students less than or equal to 100" -- seems awkward "classes with not more than 100 students" -- better, but I would say "classes with no more than 100 students" Another possibility is "classes with 100 students or fewer". Informally, many people would say "classes with 100 students or less", but purists dislike the use of "less" with countable objects.
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