It is a piece of equipment used in colorguard and winterguard. (Usually winterguard more often than colorguard)
If I were to guess, I would say they’re doing a colorguard number (since it’s fall). Especially since the students behind Brittany have their airblades marked with a piece of electrical tape- which is a common thing for performers to do in guard so that they know where they’re grabbing.
I’m also told by a someone that their old colorguard instructor is in this episode.
I cried because Brittany looked so got damn sad I can't and that look Sam gave her when Marley started singing He knew how much that song would hurt He knew Brittany would feel the sadness that has taken over all that much more and I can't SANTANA BRING YOU ASS BACK TO LIMA!