If you are a student:
MLA and APA are the two formats most commonly required in K-12 and college, with Chicago/Turabian a close third. Your teacher will tell you what format to use. If they haven't asked for a specific style, be sure to ask them before you start. Although NoodleTools does allow you to convert between citation styles, different styles sometimes require different information for certain sources. You'll likely have to double-check your citations if you convert the project later, so it is best to start out with the right one.
If you are a teacher:
Students in Humanities courses are usually asked to follow the style MLA guidelines. Students in science and research fields are usually asked to follow the APA guidelines. Chicago/Turabian is sometimes used by History or Social Studies courses. In terms of numbers, a vast majority of middle and high school students are taught MLA style, whereas in college, there is a mix, depending on the research field of the student.
In college, the primary reason for using a standardized reference format like MLA or APA is so that a professional peer (in the same discipline as the writer) can understand the syntax and relocate the writer's sources. In high school, unless the bibliography is created for a larger audience, often the only readers may be the teacher and librarian. Teachers of K-12 students typically prefer the MLA format because the MLA Handbook provides much more detail about citing books, anthologies, audiovisual material, and other sources like interviews, advertisements, and cartoons that a high school student would be more likely to use in a research paper. In contrast, the emphasis in the APA Publication Manual is on sources that students would typically only encounter in advanced research, such as technical reports, proceedings of meetings, and dissertations.
Since teaching any style at the high school level will prepare students for college documentation, the emphasis should be on why it is important to cite sources. The process of citing is similar whatever format you use - you compile a bibliography, you refer to entries in the bibliography using parenthetical references, and so forth. It is like learning a computer programming language -- once you've learned one, others follow naturally because the basic concepts (e.g., object-oriented programming) are the same, it is just the syntax and order that changes. In the case of bibliographies, most teachers do not expect their student to memorize the formatting rules; they want them to learn the reason for citing and the process of documentation. NoodleTools helps them understanding WHAT information is important to cite (which is often the same independent of the style chosen) and how to determine if they are citing correctly.