I'm in the same situation ( although for us it's more like a 20% income increase) and the soon as the first paycheck comes in, I'm going to write an email to the FA director. I'll let you know how it goes

Honesty is always the way to go. Call the school and explain. Likely you are fine this year, but next year your tax returns will reflect your increased income and fa will be adjusted.

Wow, wouldn't you want to see if the job works out first? Especially if you decide to cut back on hours.