They won't repeat a year, that can't happen for many reasons, most of which have been outlined above.
In a perfect world, they would all be on the same page when they return.
However, we knew when this home schooling started that there would be some students who just wouldn't bother with the work set, they're the same ones who don't engage and bother at school. (Only a small proportion, and I'm talking mainly years 9 upwards). This has been confirmed by the lack of work submitted thus far, despite phone calls, emails, extra help etc. When they return, they will be offered extra intervention in English and Maths, and might engage with this, but some won't. Then the exams will hit, and it will be too little, too late. But this happens every year, whether they are in school or not. I don't know if this rings any bells with teachers/TAs on here - but it's a true reflection in our school.
So I suppose my point is that the hard working students will be ok, and the ones who are not bothered will be affected. Just as it is every year.
I work mainly with SEN/vulnerable students and I worry about them being at home, from a safe guarding point of view, as they can be in school now, but we are only seeing about 20% currently in school. Some of our students are persistent school refusers, and I envisage getting students back to school, after months off, a real issue too.
This is the reality in a very big urban high school - lots of FSM students - lots of SEN. We have excellent pastoral care, rated very high in our county - but we can't help our students if they are not in school. And when they return, we will have many problems to face too.