You've rightly identified that even fictional stories must be bound in believability on some level within the confines of the universe you have created. Given that Jacksons mom and dad both knew that the boys were gay and a couple in every real sense of the word, and that they have worked tirelessly to help them be together, in a real universe similar moms and dads would know how important it would be to have pictures of those moments for memories that they could look back on later in life.
Strangely, I myself never attended a single graduation ceremony or prom during my entire education; the reason being my dad only had his holidays during specific weeks so I was always taken out of school for the last week or two and we went on family vacations instead. It never occurred to me at the time that i missed anything important, and quite frankly being a gay kid who was tortured and harassed daily at school, I had zero interest or affection for any of the students or teachers, so it was no loss felt by me. Besides, diploma's can be mailed. But that's not the universe this story is set in.