There are 109 fifth graders + 115 sixth graders + 118 seventh graders = <<109+115+118=342>>342 students going on the trip.
There are 4 teachers + 2 parents = <<4+2=6>>6 adults going on the field trip for each grade.
There are 3 grades going, so 3 grades x 6 adults/grade = <<3*6=18>>18 total adults going on the trip.
There are 342 students + 18 adults = <<342+18=360>>360 people going on the trip.
So, 360 people / 72 seats = <<360/72=5>>5 buses
#### 5