K12, Inc. offers a large number of online schools across the country. Typically, this is done as part of a charter school and in conjunction with a public school district.
There exist in the United States several chains of online schools. These can be a good choice if for no other reason than the size attests to a stability of curriculum. What, for example, you see at Colorado Virtual Academy is what you see, for the most part, at the California Virtual Academies.
I encourage you to take a look at the list of K12, Inc. online high schools available on this website.