Check out Daily Grammar, a free grammar curriculum for upper elementary and middle school. Daily Grammar consists of 440 short lessons and 88 quizzes. The lessons cover the eight parts of speech, the parts of a sentence, and the mechanics of grammar.
Writing Dynamo is an online grammar checker and proofreader that gives you detailed writing tips and feedback on voice, word choice, sentence structure, and more. Ahh this will be so helpful with senior year English papers.