It's really hard to know what you should do in this ambigious situation, since we can only know your side of it. I think the advice about trying to figure out what you can do to improve is spot on (even in situations where the boss is "out to get" you or making a "mountain out of a molehill", there are undoubtedly still areas you could improve on.)
1) I would look for a new job (assuming you are being honest about being able to work it with your school load.) When you find an acceptable new job, then give your 2 weeks notice.
2) In the meantime I would try to figure out what is wrong. Talk with your boss again, don't be whiny, accusatory, threatening (to get a new job), crying, begging, or anything like that. Just say that you've given some thought as to what he said and you want a 2nd chance to improve and show yourself capable of the work (doesn't matter if you already are capable of the work and your bosses accusation seem way off base to you, say this anyway.) Since he mentioned IV's in your first meeting, ask him how you can improve in this area (does your place of employment have any formal tech training materials as many states require, ask if you can have access to those materials to study.) Assure him that you have worked out a schedule for studying and work and that you will stick to it (if you haven't, you need to do this before talking to him.) When you make mistakes (everyone does), just say you are sorry and ask your boss or the pharmacist on duty, or even a fellow technician for advice on not making that mistake again. Be friendly with ALL your fellow employees and take ALL of their advice to heart. For example, even if you suspect you are smarter then Tech A, if you Tech A give you some advice, thank her/him and incorporate that advice (if the advice seems to be completely illogical/illegal, then talk to your boss or a pharmacist about it....if its just a mater of you not wanting to do things that way, suck it up and do it as y(as long as your co-workers are giving you advice or even "telling" you what you need to do, they care....when they stop caring at all, they won't talk to you at all.)