It can be rather daunting to see your youngest off to secondary school esp when elder siblings have had a rough ride. I certainly felt the pain on last day at primary in feeling it was the end of an era.
There is a degree of what I call 'elbows out' with some students at grammar (as I am sure there are at all schools). I had previously assumed this was due to uber confident over indulged kids who felt it was their right to have a grammar place. However, having spoken to a mum of 'elbows out' DC, I discovered their child felt they had had a terrible first year with feeling an underconfident outsider!!!
Kids all react in different ways and I have learned to be less judgemental esp in the first year. You tend to find things settle more in the subsequent years when they all mature a little and get used to each others idiosyncrasies. That said I am sure some kids cruise through y7 making friends and having laughs while excelling at everything put in front of them
They will be few and far between. Don't expect too much of your little ones, it is a big transition and so just support them as best you can. I learned not to fight every battle for them, I picked and chose which were really important (meanwhile worrying and biting my nails to the quick!). I tried to support yet allow them to find their own way in their new environment. It can be a rollercoaster at times but one which can be managed and ultimately enjoyed