I am not sure how many schools do this, but Amaroo School gives students a day off school to play in their school's Chess Championship. This has been held for the past 3 years and it normally attracts a field of between 60 and 80 players. In fact it would attract more players, but space an equipment limitations restrict the total number of entries.
The 2014 edition was held today, with 66 players taking part. Amaroo is a K-10 school, but entry is restricted to 4th grade and above. While the bulk of the entries came from the 4,5 and 6 graders, there was still a healthy entry from the high school students. My role at the event was the arbiter, and it was one of the easiest school event I have ever run in my 30 years of chess. A lot of the credit must go to Belinda Robertson, and the chess program she has run at the school over the last 6 years. All the students who took part who both familiar with the laws of chess, and more importantly, familiar with how chess tournaments are run. Before we started there was a thought we might only have time for 5 rounds, but due to the knowledgeable field, we played the full 7 rounds by 1pm, leaving plenty of time to pack up and hand out prizes.
Rarely for a school event there were no disputes, no one cried, and I only had to answer a couple of queries. I was a little bit gentle with the new FIDE 'Illegal move' loses Rapidplay rules, by implementing a '2 strikes and you're out' system, and even then players had the option of not reporting the first illegal move to me. I did get suckered by one player who innocently asked me if you could mate with K+B v K. Thinking he was about to make a draw claim I said no, but he then used this information to sacrifice his bishop for his opponents last pawn, before claiming the draw!
The winner of the event (on countback after a 3 way tie) was Vivian Lam, who also won last year. In fact each of the 3 editions of the championship have been won by a female student, which is another reason it is a rather special event.