I know that the witlesses are not supposed to go to many school activities. These include after-school sports and competitions, scouting, and other extra-curricular activities and class trips that may last beyond school hours. The most cited excuses (flimsy they may be) are bad associations, the money, time, and the possibility of compromise.
Now, I wonder how many people have missed field trips that happened during normal school hours because of being witlesses. The last I have heard, field trips (as defined as a trip that the whole grade or class is going on, starting during normal school hours or earlier, and is related to a school subject) are not explicitly banned (that, however, could change--all it takes is a single washtowel article and it becomes another no-no). The most likely excuses are bad associations (as if you weren't getting the same association in class), missing a boasting session or afternoon field circus, the money that belongs in the Worldwide Pedophile Defense Fund, and the outside possibility of its including compromise.
Note that, despite nothing being explicitly said about the subject, some parents are too self-righteous and will go beyond the things written. They just figure that, since holidays and sports are just about forbidden, they might as well do away with these field trips as well. Others are worried that the trips might not end on time (I have had a field trip where I had to be at school at 6:30 in the morning and didn't get back home until 6 PM that evening). That could cut into field circus and/or lead to a missed boasting session. And there are those who think it would "stumble" others if their children were allowed on a field trip, or that it would bother the parents' consciences.