We have found an issue with classes not showing up for subs if:
- On the day before the field trip, the class had every student in the class "attending" the field trip during that class period AND
- On the day of the field trip, one or more students' were switched to "not attending" the field trip
If both of these are true, the system doesn't realize there are now students who will need supervision by a sub.
We have a work around when this occurs, but it involves submitting a ticket to Veracross@camrelhs.org, which isn't always timely.
What you can do:
Make sure the teacher of the class is invited to the field trip in the group event, and it is marked as Invited.
If event attendance can be updated the day before the field trip, this is the best step. But we realize most changes happen the day of, right before leaving for the field trip. In that case update the student attendance to Did not Attend. 
This will ensure that the substitute will be able to take attendance for the class, and a substitute can be scheduled for the class.