Since it is a private school, some of the rules change. If they are a true private school and do not accept any State monies, they can do alot of things other schools can't do. You might ask her about setting a sign out.

My kids go to private school and in my state I can not deduct the tuition from my taxes (about $13,000/year for 2 kids). So I wash the school (church) and sidewalks and do fire extinguisher inspections as a "donation" to the CHURCH. Donations are tax detectable for the church that supports / backs the school.

In the end, I come out WAY ahead.

Maybe offer her 50% off the price and a 50% donation and a sign in the yard. This way you still get cash in your hand and also get to deduct your services on your taxes.