Pretty much this. Nobody will likely beleive you "accidentally" ingested poison. Food poisoning on the other hand happens ALL the time.
You will get sick, but be prepared for those consequences, it's not going to be a relaxing vacation you will be vomiting, diarrhea, nauseated, cramped, chronic pain, headaches, even some serious side effects, but you're intentionally doing it so you shouldn't be unaware of health issues that arise.
IMO the best way to do it is to get regular raw meat like beef or something, let it sit out for a bit(find some that will expire soon), freeze it, then let it thaw.
I however do not recommend any of this though but if you want general food poisoning it may be the best option for "getting sick" for a short period of time with serious enough health issues that warrants you to be absent from class. It's actually somewhat difficult to manually poison yourself, faking it wont get you too much time off at best, but with this you can get a doctors note. Again, this method is still iffy and can lead to serious health issues, you could always just try eating expired foods in general, I wouldn't recommend self poisoning though only because it can be more serious than expected and it's very unsatisfying.