How to not laugh inappropriatley?

Haha lol I have the same problem as you in school assembly, an asian came up and talks about racism and is saying stuff like ingrish for english and how they are equal, and I was laughing and covering my mouth and I got in huge trouble, but it is something I can't help, I doubt it is anything serious, try thinking about something else like the holocaust or something sad like that and you will probably endure through something the will get you in trouble lol