Thanks for asking.
Students can certainly work on their grammar lab lessons (or any other lesson) in their own time!
As you mention, you can only have one active lesson at a time. However, after you end a lesson, students can still see it, open it and complete the activities by going into their Study history tab on their app.
This means that, in your own time, you can start the grammar lessons you want your students to see, send the activities, close the lessons, and then ask students to open that lesson at any time.
You can also create a separate class only for the students that need the extra support, and give them the class code, so only they can see these lessons.
I hope this helps and let me know if you need anything else! If you have any questions regarding how this works, I'd be happy to arrange a call and show you.
The EF Class Team