My son went on an 8th grade D.C. trip with his school and I know how extremely intensively planned every single moment of these trips are. I would NOT just call the school and make a request unless you have done some homework and studied the agenda and logistics and then only ask if you have a very good plan to suggest that fits their schedule. Still I would be very careful not to be the difficult parent asking way too much (“Take my child to Mass, please”). I have experienced asking too much consideration for my child in school and I learned that it is typical for the teacher to be super polite and accommodating to the requesting parent’s face but then take it out on your child - like with one little subtle disparaging illusion to “your mom”, for example, which the child picks up on. One or two of these mistakes and my son did not want me talking to his teachers at all, ever again.

I did, though, but I got to treading very carefully in asking for accommodations (which I was happy to do when I was teaching, but everyone is different), and I padded my humble requests with plenty of compliments!
Probably asking for this Mass accommodation is
way too much, and could be taken as an affronting irritation - something I would not want to risk. You don’t get your way AND you are tagged as too much. A tour bus makes an expensive ride to and from Mass. And probably some teacher looking forward to his/her well-earned nap or break would be asked by his/her superior to accompany your child. And in the entire 8th grade there may be NO others who would opt to go along with your child to Mass. That’s because Sunday Obligation is
not preached from the pulpit any more - never in my son’s growing up did I ever hear it.

And being the only one being specially-accommodated this way will make your child stand out - everyone will want to know why your child is off doing something different. 8th graders do NOT like to stand out from their peers; they want to blend in. And the school staff does not like doing something that can be seen as favoring or out of the ordinary for any one child. Also if some faculty or staff does bring their own car on the trip that’s a lot to ask them, too.
I just would not ask for this accommodation at all. Its a great way to teach your child that Sunday is an obligation for Catholics, though. You can call your priest first, in case he is one of those casual “whatever” priests concerning Catholic obligations, asking if he would speak to your child (like after Mass) releasing him/her from Sunday obligation in a serious way (not discounting that it IS an obligation). And, you are not obliged to do this, but I did with my child - if for some reason we missed Mass - which was rare - we would go the next day, to a weekday morning or afternoon Mass. You could do it before the trip, if your child is all packed and rested - that takes vigilant planning ahead, though. (They come back tired!). That takes planning ahead, but all the extra work concerning Mass is on you, not the school. Maybe you can make it a positive together-experience, like a Wed. morning Mass, then out for a quick breakfast, then late to school.

That’s if he/she does not have a vital morning class, and if you don’t mind doing that. (I was never adverse to my son missing a bit of school for anything that was positive for family or for his well-being…