Khan Academy has a grammar course and not only is it free, it's really good!
Just like with ReadTheory, you can create a class and monitor your students' progress. Khan Academy works really well with Google Classroom, letting you import whole classes, allowing the students to log in using their school Google accounts. If a student already has a Khan Academy account, they can enter a class code to join your class.
Unfortunately, it's not suitable for absolute beginners, but seems to work with learners at A2 level and upwards. It provides a challenge even for the most advanced users of English.
The quizzes are short so students get feedback on their attainment and progress really quickly. It's perfect for those with short attention spans.
A difference between Khan Academy Grammar and ReadTheory is that progress is not related to any particular level of English. So as a teacher, one might consider looking at all the grammar exercises and then setting some goals for say, students aiming for "Engelska 5". Then again, the students I encounter have very different strengths and weaknesses in English grammar. So far, it's been interesting to watch them explore and discover those areas where they need to practice.