Matt,
When dealing with Mom and the Dean, remember that this is an FAI event that is the same as the Olympics in it's prizes. Your record will go into the books in the FAI library, same as say, Jimmy Doolittle. You will be awarded Gold, Silver, or Bronze Olympic Medals. It's not just toy airplanes. How many people playing football at your school ever represented their country. None. Only you.
Play the Olympic Sport card.
Chris...
Sorry, But this is a HOBBY, a pleasant past time, a distraction, nothing more. Missing school mid year or late in the Semester can be acceptable. Missing the opening days is not. It's like getting job offer and saying to your employer, Sorry but, I can't start work just yet, I've got better things to do, so you'll just have to wait. It sets a bad example. Better to say nothing to the school and show up late than tell them ahead of time that education is second tier. Of the thousands of Olympic medal winners a tiny fraction are ever heard about again. Where NFL players are household names throughout the country, and get paid tons more money to boot.
Personally as a parent, I would not have let any of my kids skip out on opening days of college, unless they were in the hospital in traction. Especially so since I was paying the lions share of the tuition. If there are school loans out, then missing the opening days may have ramifications in the processing of school loans and any grants, with the possibility of loosing any such loans or grants.
Trust me on this I have experience with this first hand. This is not to rain on anyones parade but College is serious business.
Better to put it off for a Semester completely than register and leave or register late, doing that has the potential to create all sorts of nightmares. Finally do not for one moment even attempt to compare Model Airplane competitions to any organized international competitions like the Olympics. Doing so would make a dean of admissions think you are not bright enough to attend college. It would be akin to comparing Lawn Tractor racing to NASCAR. Certainly note it under Hobbies and other interests, but make no mistake, to the rest of the world they are TOY airplanes a rather boring subject at that.