Why not? Volunteer work is to assist teacher not to replace teacher. There is some responsibility but as soon as we follow the teacher's direction it should be fine.
By the way, I went to my son's class once every two weeks.
But in the field trip, do you think the kids needs more attention? That's when my daughter's school needs volunteers, but I am afraid that what I think might not suitable for their regulation or tradition. That's my concern.
How about talk to the teacher about what they expect from a volunteer? The tradition might be different case by case. Usually I talk to the teacher every week because I pass the school everyday before school starts. Or, I think we can call the teacher.
Last year, my son's field trip is to walk to a mall. So my work is to look after him and other 3 kids from lost. They are in kidgarden so it was very easy.