Month off for Mission Trip

I am currently in nursing school for my ADN, I am in my first semester. I have worked as a CNA for a little over a year. In January I landed a great job as a CNA on the float pool at the local hospital. I love it! But before I applied at the hospital I applied to volunteer abroad to teach healthcare and I got it! It is one of my dreams to travel abroad. I talked to my boss about this and she seemed good with it and I put in my personal leave request today. My trip is 3 weeks long. Do you think it is out of line to ask for this time off when I just started? It is my dream job but also it was always a dream to travel and do good in the world! I want to tell my boss I won't give up my job for it either.
Here is my thought: Once you're a nurse, you'll be MUCH more qualified to teach.
It is very basic healthcare I should have added in, i.e hand hygiene, brushing teeth,etc
The best to have handled this situation would have been to have negotiated it as a condition of hire. In other words, as they are hiring you, get it in writing that you would be allowed the time off for this trip should the opportunity come through. People do that all the time for weddings, educational needs, etc. But it's too late for that now. They have no obligation to let you go. If you are a superstar and they REALLY want you to work for them, they might allow it. But if you live in an area where there are many equally qualified people in line to take your job, they might not. It depends on how much they need/want you.
Totally agree that this should have been mentioned somewhere along the line in your interview process, but I completely understand - you didn't want to say anything because you didn't really think you'd be selected for the opportunity.Oddly enough, it is probably easier to postpone a start date than to get any extended time off after you actually begin work... so you may get lucky. I would recommend just being completely honest. Be prepared to share the formal notification you received from the Mission organization, so your new boss can see that you received it after you had accepted the job.