Yes, you do need another form for coach trips. Just head a letter heading with the trip name and ask the parent to give their consent i.e.
I do/do not *give consent for my child ............................. to travel on the coach to
................................ on day........................ date .....................
Signed............................................ ............ Parent/Carer
Print name:............................................. ....................
Date:............................................. .........................
You could also ask for an emergency telephone number for that particular day as well.