Below are the questions that should be driving your search. Often people ask the wrong question. An example of a wrong question might be: "Where are you located?" Unless you are specifically looking for a free online school, it doesn't matter much where it is located.
So, let's get started:
1. Who accredits or approves your school? Some schools handle solely as a business and have no outside approval (beyond perhaps a business licence). You will want to know who approves the school.
2. How much does it cost? The cost to attend an online school ranges from free to more than $17,000 per year. The free options will be fairly local to you (typically a charter school in your county or in a surrounding county) and the expensive versions will be private online schools. The example of $17k is Stanford University's online high school.
3. How many credits will you accept in transfer? The answer to this question is key because how many credits they will accept plays a role in completion time for the program. Some schools will accept almost nothing while others will accept everything. It is likely that the school will need to analyze your high school transcripts to determine what is acceptable. However, if you have never attended high school, then this may not be an important factor.
4. Will I have a teacher? You will most definitely have a teacher of some sort. This question is really more about how you will interact with that teacher. Some schools require students to "attend class" weekly where you and the other students are provided with a virtual lecture. Others do not and the teacher mostly just grades your completed duty. The rest are somewhere in-between. Decide which works best for you.
5. Will I work with other students? Some students, typically the younger ones, want to work with other students. Some students, typically the older ones, do not. Figure out if this is important to you. A related question is how you can interact with other students, but not in a virtual classroom setting. Perhaps they have an online forum set up where students can discuss classes and create relationships.
6. Where do your graduates end up? Do most of the school's graduates join the work force or do most go to college? Someone at the school should be tracking this information (and, if they are not, this should be a point of concern). There is no right or wrong answer here, but you want to align your goal with where most of the students end up. If most go to college and you want to go to college, this school may work for you.
7. How long have you been "in business?" Longevity breeds stability. Or so it is hoped. Find out how long it has been in existence. Don't forget, though, that sometimes "new" can mean "innovative."
8. How long will it take for me to finish? This is related to question #3 above, but goes beyond that point. Some programs march you through step by step and it is hard to accelerate to graduate early, while others are specifically set up for the student who wants to finish in less than four years.
9. What extracurricular activities are available? Unlike the past, online schools now sometimes have proms, field trips, graduation ceremonies, and so on. Decide how much this sort of thing is worth to you (and how likely you are to take part given where you are located and where the school is located).
10. How do I turn in assignments? This really only becomes important if you are located outside the United States. If you have to fax in assignments (an increasingly smaller group of schools require this), then phone charges come into play. Also, fax machines are not readily available everywhere in the world (and, yes, I am talking to you, missionary kids!).
It is important for you to understand that these questions are a starting point. They are to give you initial guidance. It's likely that you will have follow-up questions for the school after hearing some of their answers. And you should.