Tipping for Charter Bus

School trip, mix of half adults and half teenagers and we hired a bus to provide airport transportation. The drive is about 3.5 hours and we leave very early in the morning (3 am) and arrive back at like 1030 at night. 44 passengers. Should I tip our driver by headcount or percentage? The total cost of the bus round trip is $5500 so a percentage tip seems like a lot.