I think that it depends on if it is caused by a medical condition such as dyslexia or ADD or if the child is just slacking off. If it is a medical condition, I dont think that punishment of taking away the privilege to go on a trip is appropriate. On the other hand, if they are just slacking off I believe that it is absolutely appropriate to not let them go.
Is it an educational trip? Then yes. It might spark the learning process if they can see how what they are applies. If it is a fun trip it all depends on why they aren't preforming. If the child is actually trying yes, let them go. If they are slacking, no way.
If the school trip is educational, you can still take them for they might learn something beneficial during the trip. However, if it is just an ordinary school trip, do not let them go until they prove they can work hard next time and pass their exams.