My perspective is that you don’t even need a school that offers a composition focus. It depends on what you want.
Most general music major will include composition. Some will have lessons or at least small tutorials. Some will have undergrad composer forums weekly (or less). Some will arrange for performances in the music dept. some will have them as extracurriculars, and some will not and students have to organize their own. There are lots of things to look for on websites and visits, and questions to ask. But many schools will be able to provide what you need to go on to a top grad program if that is what you want.
Brown has a composition strand so add that to the list above.
Are you interested in other academic areas? Do you want a school with distribution requirements or freedom to choose your courses? Brown, Amherst, Bennington and Sarah Lawrence are relatively free in choice, maybe others. Another thing to look at so that you can take more music classes if you like.
To be honest, if you look at grad students at top grad programs, they come from all kinds of schools, including state U’s (which usually have BM programs) and small schools I have never heard of.
Some composers major in music and formally focus on composition. Some don’t even major in composition and have a teacher. Some don’t even have a teacher. There is a wide range.
Also think about whether the presence of grad students in a school is plus or minus. Often you end up being taught by a grad student and most opportunities go to them, but not always.
Finally, the presence of a BM program can sometimes detract from opportunities for BA students. Not always but that is something to ask about. Do the best teachers and performance opportunities go to the BM students?
If you are a person who needs a teacher, look for a professor whose work you like or whose students’ works you like. It is important to find diversity in student’s works so that you can be sure their individuality is being honored.
Think about summer programs as well. Does the school fund them?
I would find schools that you like based on the usual criteria, such as size, location, academics, and “vibe.” Then look at the music websites. Check out curriculum, actual course descriptions, performance opportunities, access to lessons and whether they are paid, credit for lessons and performance and so on.
Good luck!