So understand your fear Spooked parent.
My kids teachers now almost expect to see me whenever a school trip is looming!
For my own sanity I (privately, without the kids knowing, to save their embarrasment)go to have a word with the teacher and get them to explain to me any of the bits I'm worried about.
DS started to have swimming every week at school this term and before it started I would imagine how he would run and fall, knock himself unconcious, fall in unnoticed and.........
Crazy, but some of us are born worriers. I do consciously make sure I don't spread my fear onto my kids and do all the background work without them knowing, so that they don't carry any of the worries.
Anyway, re the swimming, the teacher told me how regimented it is, how it has to be, so precisely that sort of thing doesn't happen, and I felt fine about it all them.
Please go and speak with the teachers and get them to go over any aspect you're not happy with.
Its your son and it's only right you want to ensure he is safe so don't worry about asking, and then, hopefully, you can wave him off with a smile on your face.