Take awesome, free writing courses on Coursera. These can help prepare you for high school or college writing classes by helping you build the necessary foundations in grammar, logic, argument, structure, etc.
PHschool offers wonderful online textbook resources for grades 6-12. Here, you'll find explanations, lessons, tests, quizzes, and writing prompts that can aid tremendously in terms of English literature, writing, and grammar.
Though you should have access to online databases of scholarly and peer-reviewed journals through your current school's library, Google Scholar is a great supplement. Here, you'll find educational articles about countless topics.
Google Drive is both a great online locker and a method for collaboration. You'll be able to download free programs like Word and Photoshop, and share documents with anyone. You can even edit documents simultaneously.
Grammarly is a paid-membership service that helps you check that your grammar is correct. You simply upload any amount of text into the system, and it'll complete a thorough grammar-check based on Standard Written English.