I obviously know a little bit about your business and the community you serve. You guys have been running these classes free for a little bit of time now and so my advice would would be to push it, but instead of pushing it hard, push it soft! Have a 15sec little audio bite that explains your situation, that like so many businesses you’re doing your best to survive with no clear view on when the current lockdown will end. Repeat variations of that message 3 times throughout the class like “don’t forget to give that donate button a fist bump”. Your community love you, I’m sure they won’t be offended if you give it a nudge, at the end of the day, if they don’t want to donate they won’t, so there’s nothing to lose by asking in the right way.