I understand how badly you must feel, but schools, agencies, even places of employment, are afraid of the responsibilty, liabilities, and legal stuff, regarding the mental and/or physical state of a students, employees, etc. I know that doesn't make it right or wrong, but in a day of everyone sueing everyone, companies, schools, and other facilities don't want to take risks.
Example, if a person is suicidal, takes meds. other than prescribed by docs., or physically handicapped, many trips or outings, can't always support special needs, not being qualifed or even certified to care for people with special needs, this may be a reason why you were made not to be allowed to attend the trip. I am not taking their side but I'm explaining why this refusal was made.
If it is not too late, you can try to get a doctor's note stating you are physically/psychologically able to attend this trip, then they may reconsider, and allow you to go.
One thing, they should reimburse you for the fee you paid for this trip.
Now many schools do have some sort of set policies if a student has caused some problem, such as bad behaviour, not abiding by school policies, they most likely get the final say so. Not knowing what had happened for them to decide you cannot go, makes it hard to come to a valid idea.
By any chance did you have a run in with a teacher or staff? Did you miss a lot of school days? Was any reconciliation made? These are some issues that may have been a factor in denial of your attendance.
If there are no valid reasons, then I would suggest your parents take it up with the school and demand some explanation why the school decided you should not go.
Right now it is hard, but keep in mind there will be other trips, if not through school, through clubs,church group trips, and possible trips with your family.
I wish you some closure on this sad experience.

Take care,
DE