I'd suggest starting with major and minor scales for 6th grade trumpeters. Because they're most likely just starting out maybe begin with the keys with the least amount of sharps and flats. Then, once they're comfortable with that, proceed with the rest.
When I first started playing music I only learned my major and minor scales in a few keys; the easy ones. Once I was in college I had to bear down and get fluent with the rest of them very quickly if I was going to learn anything. I wish that I would have learned all keys at a young age.
Here are lists of those scales in order of the circle of fifths: