Was this one of those all-day-long events where groups of parents and other audience members come and go, perform and are judged but then depart if they don't advance? The kind of event where High School Band X is playing at around 9:30 a.m. and Band Y might not play until 3:30 p.m., Band Z later still, so the audience ebbs and flows all day long?
Not clear if that's the case here or if folks were getting seats very early for just one show in the evening. But if the former is the case, well, I'd be peeved if the band I wanted to see was playing in one hour's time and I'm there to get a seat, but the other people are there to see a band playing much later in the day and they are there hours and hours early to save seats for that performance -- thus denying seats to people who want to see bands that will perform earlier.
If that were the case, I'd probably ask, "Oh, are you here to see Band X that is playing in an hour? No? What time is your school's performance?" and then I'd politely but pretty firmly say that we'd like to sit there in order to see our band's show and will leave when it's over, which is well before their band's show.
Or maybe these parents stay all day long and expect to see the big finale on the assumption their kid's band will make it into that slot? If that's their case, then they aren't going to listen to even a polite request, unfortunately.
In your case it didn't really affect you personally but I would still be sitting there thinking how rude it was as person after person looked at the row and asked, "Uh, can we sit there?" while the seat-savers keep saying no and the only gap in the stadium gets more and more obvious as the place gets busier.