Heres what I'd do and, in fact, what I have done in my younger brothers class: Go up to whomever is at the desk as ask if you can clean out the vase while your waiting. Mention you keep bettas at home and try and strike up a friendly conversation about them. If they ask questions and seem interested, take the opportunity to kindly educate them. If they don't seem interested, ask a few questions about the betta yourself and feel it out.....and if the right opportunity arises, you could offer to take him home.
I've developed a good relationship with my brothers teacher over their classroom betta and she seems much more curious and willing to learn then I first anticipated. Some people are just misinformed, you may be surprised how much they care after they learn ;)