Was the school small enough for 9th and 12th graders to share a lunch hr

if the school is not so big, then they probably have only 2 periods for lunch a day, and kids from all grades would use that period for their lunch as dictated by their other class scheduling priorities. Its very probable that this happens in many schools.

I went to a large school and the 5 or so lunch periods all had a mix of people from each grade. They had to do it that way because there was no guarantee that every class that was offered would only have students from a single, or even two different grades. Some students could be ahead or behind and some classes were offered to all grades.

My middle school had 3 lunch periods, but my high school only had one lunch period. There was a huge cafeteria and you were also allowed to eat in the hallways surrounding the cafeteria, so there was plenty of room.

My school had about 1600 students and only had two lunch periods.

My school had about 2400 students and we had 4 35-minute lunch periods. Each lunch period was given to a different class subject (one for math classes, one for English and English-related classes (i.e. drama, journalism, etc.), one for social studies and foreign language classes, one for science/art classes or any class that required like a lab or workshop) and included students of every grade, so it was a good opportunity for students of different grades to interact with each other.