Oh, please. I am a teacher in the Merrillville system, and this is yet again another 'concerned' (and yet, uninformed) taxpayer griping.
Merrillville does indeed have its own bus fleet. It is used to transport the children to and from school, and the buses are shared by all the schools, which is why they are on staggered schedules. When a field trip is planned, it must fit into the existing schedule-- in other words, that bus must leave after all students have been dropped off at school, and return before students are dismissed. There are a few "extra" buses, but when there are many field trips on one day, they must use another contractor. This is especially true when students are going somewhere in Chicago, as the day will be longer.
This cost is passed on to the students attending the field trip, not to taxpayers. It usually works out to an extra dollar or two per child. And, lest anyone worry their child is being robbed of an extra dollar, field trips are not mandatory, and for some children, fees can be waived.
I wonder, did you call the school and ask about the bus situation, or did you simply see a Laidlaw bus in front of the school and assume?