DS1 (15) has been in and out of school for about four years, Egg, as he has special needs and couldn't cope in mainstream. His education ended up being very patchy, as I home educated him for a year, then he went back to school, then he came out again and was educated by the LA for about six months, then home educated for a year, then back to school again... and on and on until I finally got him a statement and funding to go to a school for children with special needs in September. If I had set out to home educate him without interruption, he might have had a more consistent education, but it's easy to say that now, with the benefit of hindsight.
However, in spite of that, he is slotting in well at his new school and has been placed in the top groups for maths and English. I don't understand how because he hasn't done any formal study of either for a long time. This supports what autonomous home educators say, ie. children continue to educate themselves even if they don't go to school or having any formal teaching. For months at a time, DS1 couldn't leave the house because of chronic anxiety (he has Aspergers), so he ended up reading online (he won't read books anymore, much to my frustration). Through this, he has become interested in politics, current affairs, religion and much more. He has essentially been teaching himself.
I was most concerned about maths, as he hasn't been following any regular study of maths recently, but he seems fine with it and what he doesn't know he has plenty of time to learn before his GCSE, as it is a two-year study course.
My younger son (9) was only out of school for one year. I took him out because he was miserable and we had found a better school for him, but the place fell through. I have yet to hear what the teacher says about him (meeting next week), but he is very happy at his new school.
The woman who came to see us from the LA, after I deregistered DS2, said that children's educational needs vary so widely as they progress through school that it really doesn't matter if they are behind when they arrive because schools are set up to deal with this.