Is it normal to get erections doing tai chi in class?

Not usual: Suggest you try wearing jockey or tight underwear.

Yes: Whenever a man is doing something that's physical, fulfilling and makes him feel really good about himself, he's likely to get all excited. It's different for different guys -- anything from riding the subway to singing the national anthem can get things going. All guys know about this stuff. Accept who you are ; be glad that wholesome things turn you on. And wear a good supporter for concealment.