Both of mine went in at 6th form level so may be a bit irrelevant but it's worth me telling you the good and bad!
Friendships, no worries whatsoever. Both made friends really easily - more easily than their schooled peers who found the shake up of suddenly being in with different people for 6th form hard to deal with. Both have had comments "we'd never have believed you were home educated" and DD1 had "but you're so normal!" . DD2 is the one I would have worried over as she's a little more quirky and struggled with friendships during her brief time in primary school - but has had no issues whatsoever. I think home ed has really helped her in this area.
Academically, no worries. Both had been educated at home through GCSEs/iGCSEs with a combination of self study and me helping. Both have coped with A levels as well as their peers. DD1 got good A levels and is currently at full time performing arts school in London having a blast. DD2 is doing A levels in Physics, Maths and Biology (and holds Psychology AS which we actually did as home ed) and looks set to do well and go on to a good uni.
Only issue we had was DD2 going into a school 6th form and coping with the pettier of their rules. If she thought something was pointless, she would say so, and this got her into some trouble. For example she was told off for wearing a dress with doc martens. Others were wearing dresses and doc martens, but not in the same outfit. She kept pushing them to clarify what exactly was wrong with it and as they couldn't, the teacher got flustered and just punished her anyway. She found these kinds of things hard to take and do was pretty unhappy by the end of the year. She also refused to go to things she thought were wasting her time such as some of the PSE lessons, or if she thought she could make better use of her time studying at home, and that landed her in trouble too. She expected the teachers to reason with her as though she was an adult and couldn't cope with it when they treated her as though her view was not valid because she was the pupil.
I must add that we have suspected all her life she is on the spectrum, but have never had her tested for Aspergers as she doesn't want to be (though she recognises it in herself). This year she has transferred to college where they don't do "rules for rules sake" and she is very happy and it's all going very well indeed.