Can I use the airport lounge?

I'm traveling to St Marteen, but I have a layover in Miami for 6 hours. I'm flying first class on American Airlines, so I assume I was able to go in the lounge. I looked it up to make sure, because I'm going next month, but it said international places, which I'm going to. The caribbean was not on the list of places though. So am I not able to use the lounge? Can someone tell me how this works.

Will I need a second seat on an airplane?

My grade 12 band class is going on a summer break field trip to Ontario next week but I'm worried I might need a 2nd seat. I read a PDF on airplane seats and apparently they range from 17-18 inches and my "surface measurement" is around 23-24 inches. Would the airline be a little more lenient since its a whole class trip? I could use some feedback because I might just end up not going. It would be super embarrassing if I had to get a second seat in front of my class.