Do I need liability insurance if I only teach one class?

I normally teach group fitness classes in a gym where I’ve covered under their insurance, but I’ve been asked by one of my participants if I could teach ONE class to his volleyball players after they finish a 6-week series of agility drills, just for something fun and different for them. It would be in a gymnasium-type setting, not my gym, and it would be a one-time thing, and all of his players will have previously signed waivers before starting 6-week session with him.

Do I need to purchase professional liability insurance for myself? Is there a one-time coverage I could get, because I don’t plan on teaching classes outside my gym (just this once). Thanks!