Is it rare to play a high b flat on trombone in eighth grade?

Yes, it is relatively rare for an eighth-grade trombonist to be able to play a high B flat. Depending on the grade level and the skill level of the trombonist, they might play as low as low B flat or as high as high C, but high B flat is unusual for an eighth grader to play reliably because it is in the middle of the high register.