Just my two cents...My boy finished year 9 (equivalent to form 2 in local school) in ESF in HK (born in early 2007) and he is now in year 10 in a private school in U.K. According to him, he has slight difficulty studying chemistry and geography initially due to differences in syllabuses in UK/HK and he needs to catch on up these, by asking teachers to help out. I guess it could sometimes help train the kids to be more proactive. He has a smooth transition in learning the foreign language which he has been learning in ESF in HK.
I guess teenagers would be able to cope with times. But of course, it could alleviate your / your kid's stress if you can get hold of the school's year 9 syllabus beforehand and prepare in the summer.
我一個year10过去,一個year9過去,兩個都話有d課程係香港冇cover嘅,但要追並不難,反而year9过去学校多d活動去幫佢融入,大家都新入學,容易揾friends, year 10個個因為人哋己讀一年,要花小小時間揾朋友,好在兩個都話入sport team最快揾到朋友,teamates仲互相照顧