Purdue University's Online Writing Lab (OWL)
This site is put together by Purdue University, so some of its grammar and writing resources are targetted at university students, but there are great resources for high school students too. Some information is most useful for students new to writing in English. They also have sections dedicated to teaching writing mechanics (sentence-level organization), grammar and punctuation.
University of Ottawa's HyperGrammar Online Course
This site is put together by the University of Ottawa's Writing Centre and covers the same material as their first-year English grammar course. Don't let the fact that it is basically a university course scare you; they are covering the basics of good English and what they cover will be useful to high school students too.
Khan Academy for Grammar
Khan Academy is famous for helping people finally "get" tough math concepts, but it's not just for math any more. One of their many new subject areas is grammar. Learn the basics or just get some extra practice.
This is a Canadian site that has a free downloadble workbook that will teach you the basic foundations of grammar. It will teach you about the parts of speech and how to use them correctly (subject, verb, adjective, adverb, etc.), how to avoid misusing commonly confused words (e.g. their/there/they're) and how to combine simple sentences into more complex sentences without making grammatical errors.
Free English Resources from the Canadian Government
The Canadian government has numerous free online resources for learning how to use the English language correctly. Whether you are struggling with proper punctuation, grammar and/or vocabulary, this site will have a resource to help you work on your problem area.
This site is American, so ignore all those funny spellings (color, labor, etc.). It is useful for looking up many common grammar mistakes and learning how to write things more correctly. For those who are struggling with many areas of grammar, there is also a free online course that will help you improve your writing.
No Red Ink
While this site is designed, in part, for teachers to use with their entire class, students can sign up for an account and work through the activities without having a course code. This site is also helpful because it will help you with not only grammar, but also using MLA style to document your research correctly.