I went through school in the 70s and 80s and always had school lunch. I hardly saw anybody bring lunch to school. Maybe it was because of the schools I went to, but they were public schools. They were "good" schools though (some of the best in the major metro area), so maybe that changes things. In high school I honestly don't remember seeing anybody bring lunch, obviously people did, but it would have definitely been the super minority and definitely not common by any means.