The convention fee should be part of professional development - which is another allowable budget item in my opinion.
I know I am very fortunate this point though to have a budget that can cover that easily. My first school wasn't that way and I was stuck paying their state dues and NAFME's chunk too. I know some schools don't have the budget for it, but I do truly think this IS justified as not being a personal expense. Worth pushing the district to cover.