Turn on attendance first. It's the module every teacher touches daily, and it's the fastest way for staff to build a habit around the new system.
Publish your fee structure early, even before enabling online payments — visibility into dues alone reduces the number of "how much do I owe?" calls to your office.
Send one clear message to parents explaining what the parent app does for them specifically — real-time attendance, fees and homework — rather than a generic "new system" announcement.
Pick two or three teachers to be early champions who can help answer colleagues' questions during rollout.
Review your first month's attendance and fee reports together as a staff meeting — seeing the data in one place is often what convinces the last few skeptics.