XP assignments are fundamentally flawed, for exactly this reason. I would go for a combination of skill assignments, so you can insist that they progress to a particular point in the course, and then I would set them the homework of keeping the tree gold, and get them to show you on their phones / computers that they've done so.

Instead of assigning a certain amount of XP, you could assign specific lessons/skills that need to be completed in the two week period. Other than that, I'm not really sure...

Give them an F if they do it again.

Maybe what is needed is the ability to block individual students (or even entire classes, if it cannot be done on an individual basis) from particular lessons.

It would be best if we could block either an individual student from particular lessons or block the entire class from a particular lesson.

This kind of control is really needed for this to be an effective platform for instructors. As a parent of teenagers I can tell you that their greatest motivation seems to be to avoid anything that involves mental effort. Unless it's something they think they're not supposed to be doing. Then they can do amazing things.