Go to your professor and tell him/her that you suffer from anxiety and if it's possible to do the presentation in front of him/her during lunch or after class. If he/she says no, then go to your doctor and ask for fast acting anxiety meds for this particular occasion, explain to your doctor about the presentation and you need something for anxiety for that day, usually they will give you a fast acting anxiety pill. You have a few options, so don't drop out. Taking your time with a presentation also helps, rushing through something just builds the anxiety so remember to take all the time you need and breathe and pause. Beta blockers don't act fast, they are a kind of medication that needs to build up in your system and they don't necessarily work for individual moments of stress. Valium does, that's one that comes to mind. Valium is the pill that lots of people take right before a plane ride because they're nervous and because it's fast acting.